上一節介紹了PostgreSQL如何解析查詢語句中的表示式列並計算得出該列的值,也瞭解了PG透過執行一系列的步驟對錶達式進行解析最終得到表示式列值.本節介紹了PG如何構建表示式解析步驟,這是第一部分,介紹了ExecInitIndexScan->...->ExecBuildProjectionInfo函式的實現.
每一個ExecInitXXXScan節點都會呼叫ExecBuildProjectionInfo函式,用於根據targetlist構建投影(Project)資訊,構建投影資訊的時候構建表示式解析步驟.

### 一、資料結構
**EEO_XXX宏定義**
opcode分發器宏定義
```
/*
 * Macros for opcode dispatch.
 * opcode分發器宏定義
 *
 * EEO_SWITCH - just hides the switch if not in use.
 * EEO_SWITCH - 如未使用,則隱藏switch
 * 
 * EEO_CASE - labels the implementation of named expression step type.
 * EEO_CASE - 標籤化已命名的表示式步驟型別的實現
 * 
 * EEO_DISPATCH - jump to the implementation of the step type for 'op'.
 * EEO_DISPATCH - 跳到'op'指定的步驟型別的實現
 * 
 * EEO_OPCODE - compute opcode required by used expression evaluation method.
 *            - 透過請求的表示式解析方法計算opcode
 * 
 * EEO_NEXT - increment 'op' and jump to correct next step type.
 *          - 'op'++並跳轉到下一個步驟型別
 *
 * EEO_JUMP - jump to the specified step number within the current expression.
 * EEO_JUMP - 在當前表示式中跳轉到指定的步驟編號
 */
#if defined(EEO_USE_COMPUTED_GOTO)
//--------------- 定義了EEO_USE_COMPUTED_GOTO

/* struct for jump target -> opcode lookup table */
//跳轉target -> opcode搜尋表結構體
typedef struct ExprEvalOpLookup
{
    const void *opcode;
    ExprEvalOp  op;
} ExprEvalOpLookup;

/* to make dispatch_table accessible outside ExecInterpExpr() */
static const void **dispatch_table = NULL;

/* jump target -> opcode lookup table */
static ExprEvalOpLookup reverse_dispatch_table[EEOP_LAST];

#define EEO_SWITCH()
#define EEO_CASE(name)      CASE_##name:
#define EEO_DISPATCH()      goto *((void *) op->opcode)
#define EEO_OPCODE(opcode)  ((intptr_t) dispatch_table[opcode])

#else                           /* !EEO_USE_COMPUTED_GOTO */
//--------------- 沒有定義EEO_USE_COMPUTED_GOTO

#define EEO_SWITCH()        starteval: switch ((ExprEvalOp) op->opcode)
#define EEO_CASE(name)      case name:
#define EEO_DISPATCH()      goto starteval
#define EEO_OPCODE(opcode)  (opcode)

#endif                          /* EEO_USE_COMPUTED_GOTO */

#define EEO_NEXT() \
    do { \
        op++; \
        EEO_DISPATCH(); \
    } while (0)

#define EEO_JUMP(stepno) \
    do { \
        op = &state->steps[stepno]; \
        EEO_DISPATCH(); \
    } while (0)

```

**ExprState**
解析表示式中執行期狀態節點
```

/* Bits in ExprState->flags (see also execExpr.h for private flag bits): */
/* expression is for use with ExecQual() */
#define EEO_FLAG_IS_QUAL                    (1 << 0)

typedef struct ExprState
{
    //節點tag
    Node        tag;
    //EEO_FLAG_IS_QUAL
    uint8       flags;          /* bitmask of EEO_FLAG_* bits, see above */

/*
     * Storage for result value of a scalar expression, or for individual
     * column results within expressions built by ExecBuildProjectionInfo().
     * 儲存scalar expression表示式
     *   和透過ExecBuildProjectionInfo()函式建立的expressions單列的結果.
     */
#define FIELDNO_EXPRSTATE_RESNULL 2
    bool        resnull;
#define FIELDNO_EXPRSTATE_RESVALUE 3
    Datum       resvalue;

/*
     * If projecting a tuple result, this slot holds the result; else NULL.
     * 如果投影元組結果,該slot儲存結果,或者為NULL.
     */
#define FIELDNO_EXPRSTATE_RESULTSLOT 4
    TupleTableSlot *resultslot;

/*
     * Instructions to compute expression's return value.
     * 計算表示式返回結果的基礎"架構"
     */
    struct ExprEvalStep *steps;

/*
     * Function that actually evaluates the expression.  This can be set to
     * different values depending on the complexity of the expression.
     * 實際解析表示式的函式.
     * 根據表示式的複雜程度,可以設定為不同的值.
     */
    ExprStateEvalFunc evalfunc;

/* original expression tree, for debugging only */
    //原始的表示式樹,僅用於debugging
    Expr       *expr;

/* private state for an evalfunc */
    //evalfunc的私有狀態
    void       *evalfunc_private;

/*
     * XXX: following fields only needed during "compilation" (ExecInitExpr);
     * could be thrown away afterwards.
     * XXX: 接下來的欄位在"compilation" (ExecInitExpr)期間需要,之後可被"扔掉".
     */
    //當前的步數
    int         steps_len;      /* number of steps currently */
    //steps陣列已分配的長度
    int         steps_alloc;    /* allocated length of steps array */
    //父PlanState節點(如存在)
    struct PlanState *parent;   /* parent PlanState node, if any */
    //用於編譯PARAM_EXTERN節點
    ParamListInfo ext_params;   /* for compiling PARAM_EXTERN nodes */
    //
    Datum      *innermost_caseval;
    bool       *innermost_casenull;

Datum      *innermost_domainval;
    bool       *innermost_domainnull;
} ExprState;

```

**ExprEvalStep**
表示式解析步驟結構體
```

typedef struct ExprEvalStep
{
    /*
     * Instruction to be executed.  During instruction preparation this is an
     * enum ExprEvalOp, but later it can be changed to some other type, e.g. a
     * pointer for computed goto (that's why it's an intptr_t).
     * 待執行指令.
     * 在指令準備期間這是列舉型的ExprEvalOp,
     *   但後續會被改變為某些其他型別,比如用於goto的指標,因此被定義為intprt_t型別
     */
    intptr_t    opcode;

/* where to store the result of this step */
    //儲存該步驟的結果
    Datum      *resvalue;
    bool       *resnull;

/*
     * Inline data for the operation.  Inline data is faster to access, but
     * also bloats the size of all instructions.  The union should be kept to
     * no more than 40 bytes on 64-bit systems (so that the entire struct is
     * no more than 64 bytes, a single cacheline on common systems).
     * 操作的內聯資料.
     * 內聯資料用於更快的訪問,但同時會導致指令的盤膨脹.
     * 聯合體在64-bit系統上應保持在40位元組範圍內
     * (因此整個結構體不應大於64位元組,普通系統上的單個快取線大小)
     */
    union
    {
        /* for EEOP_INNER/OUTER/SCAN_FETCHSOME */
        //用於EEOP_INNER/OUTER/SCAN_FETCHSOME
        struct
        {
            /* attribute number up to which to fetch (inclusive) */
            //獲取到的屬性編號
            int         last_var;
            TupleDesc   known_desc;
        }           fetch;

/* for EEOP_INNER/OUTER/SCAN_[SYS]VAR[_FIRST] */
        struct
        {
            /* attnum is attr number - 1 for regular VAR ... */
            //attnum是常規VAR的attr number - 1
            /* but it's just the normal (negative) attr number for SYSVAR */
            //對於SYSVAR,該值是常規的attr number
            int         attnum;
            Oid         vartype;    /* type OID of variable */
        }           var;

/* for EEOP_WHOLEROW */
        struct
        {
            Var        *var;    /* original Var node in plan tree */
            bool        first;  /* first time through, need to initialize? */
            bool        slow;   /* need runtime check for nulls? */
            TupleDesc   tupdesc;    /* descriptor for resulting tuples */
            JunkFilter *junkFilter; /* JunkFilter to remove resjunk cols */
        }           wholerow;

/* for EEOP_ASSIGN_*_VAR */
        struct
        {
            /* target index in ExprState->resultslot->tts_values/nulls */
            int         resultnum;
            /* source attribute number - 1 */
            int         attnum;
        }           assign_var;

/* for EEOP_ASSIGN_TMP[_MAKE_RO] */
        struct
        {
            /* target index in ExprState->resultslot->tts_values/nulls */
            int         resultnum;
        }           assign_tmp;

/* for EEOP_CONST */
        struct
        {
            /* constant's value */
            Datum       value;
            bool        isnull;
        }           constval;

/* for EEOP_FUNCEXPR_* / NULLIF / DISTINCT */
        //對於EEOP_FUNCEXPR_* / NULLIF / DISTINCT
        struct
        {
            //函式的檢索資料
            FmgrInfo   *finfo;  /* function's lookup data */
            //引數資訊等
            FunctionCallInfo fcinfo_data;   /* arguments etc */
            /* faster to access without additional indirection: */
            //無需額外的指向,更快速的訪問
            PGFunction  fn_addr;    /* actual call address */
            int         nargs;  /* number of arguments */
        }           func;

/* for EEOP_BOOL_*_STEP */
        struct
        {
            bool       *anynull;    /* track if any input was NULL */
            int         jumpdone;   /* jump here if result determined */
        }           boolexpr;

/* for EEOP_QUAL */
        struct
        {
            int         jumpdone;   /* jump here on false or null */
        }           qualexpr;

/* for EEOP_JUMP[_CONDITION] */
        struct
        {
            int         jumpdone;   /* target instruction's index */
        }           jump;

/* for EEOP_NULLTEST_ROWIS[NOT]NULL */
        struct
        {
            /* cached tupdesc pointer - filled at runtime */
            TupleDesc   argdesc;
        }           nulltest_row;

/* for EEOP_PARAM_EXEC/EXTERN */
        struct
        {
            int         paramid;    /* numeric ID for parameter */
            Oid         paramtype;  /* OID of parameter's datatype */
        }           param;

/* for EEOP_PARAM_CALLBACK */
        struct
        {
            ExecEvalSubroutine paramfunc;   /* add-on evaluation subroutine */
            void       *paramarg;   /* private data for same */
            int         paramid;    /* numeric ID for parameter */
            Oid         paramtype;  /* OID of parameter's datatype */
        }           cparam;

/* for EEOP_CASE_TESTVAL/DOMAIN_TESTVAL */
        struct
        {
            Datum      *value;  /* value to return */
            bool       *isnull;
        }           casetest;

/* for EEOP_MAKE_READONLY */
        struct
        {
            Datum      *value;  /* value to coerce to read-only */
            bool       *isnull;
        }           make_readonly;

/* for EEOP_IOCOERCE */
        struct
        {
            /* lookup and call info for source type's output function */
            FmgrInfo   *finfo_out;
            FunctionCallInfo fcinfo_data_out;
            /* lookup and call info for result type's input function */
            FmgrInfo   *finfo_in;
            FunctionCallInfo fcinfo_data_in;
        }           iocoerce;

/* for EEOP_SQLVALUEFUNCTION */
        struct
        {
            SQLValueFunction *svf;
        }           sqlvaluefunction;

/* for EEOP_NEXTVALUEEXPR */
        //EEOP_NEXTVALUEEXPR
        struct
        {
            Oid         seqid;
            Oid         seqtypid;
        }           nextvalueexpr;

/* for EEOP_ARRAYEXPR */
        struct
        {
            Datum      *elemvalues; /* element values get stored here */
            bool       *elemnulls;
            int         nelems; /* length of the above arrays */
            Oid         elemtype;   /* array element type */
            int16       elemlength; /* typlen of the array element type */
            bool        elembyval;  /* is the element type pass-by-value? */
            char        elemalign;  /* typalign of the element type */
            bool        multidims;  /* is array expression multi-D? */
        }           arrayexpr;

/* for EEOP_ARRAYCOERCE */
        struct
        {
            ExprState  *elemexprstate;  /* null if no per-element work */
            Oid         resultelemtype; /* element type of result array */
            struct ArrayMapState *amstate;  /* workspace for array_map */
        }           arraycoerce;

/* for EEOP_ROW */
        struct
        {
            TupleDesc   tupdesc;    /* descriptor for result tuples */
            /* workspace for the values constituting the row: */
            Datum      *elemvalues;
            bool       *elemnulls;
        }           row;

/* for EEOP_ROWCOMPARE_STEP */
        struct
        {
            /* lookup and call data for column comparison function */
            FmgrInfo   *finfo;
            FunctionCallInfo fcinfo_data;
            PGFunction  fn_addr;
            /* target for comparison resulting in NULL */
            int         jumpnull;
            /* target for comparison yielding inequality */
            int         jumpdone;
        }           rowcompare_step;

/* for EEOP_ROWCOMPARE_FINAL */
        struct
        {
            RowCompareType rctype;
        }           rowcompare_final;

/* for EEOP_MINMAX */
        struct
        {
            /* workspace for argument values */
            Datum      *values;
            bool       *nulls;
            int         nelems;
            /* is it GREATEST or LEAST? */
            MinMaxOp    op;
            /* lookup and call data for comparison function */
            FmgrInfo   *finfo;
            FunctionCallInfo fcinfo_data;
        }           minmax;

/* for EEOP_FIELDSELECT */
        struct
        {
            AttrNumber  fieldnum;   /* field number to extract */
            Oid         resulttype; /* field's type */
            /* cached tupdesc pointer - filled at runtime */
            TupleDesc   argdesc;
        }           fieldselect;

/* for EEOP_FIELDSTORE_DEFORM / FIELDSTORE_FORM */
        struct
        {
            /* original expression node */
            FieldStore *fstore;

/* cached tupdesc pointer - filled at runtime */
            /* note that a DEFORM and FORM pair share the same tupdesc */
            TupleDesc  *argdesc;

/* workspace for column values */
            Datum      *values;
            bool       *nulls;
            int         ncolumns;
        }           fieldstore;

/* for EEOP_ARRAYREF_SUBSCRIPT */
        struct
        {
            /* too big to have inline */
            struct ArrayRefState *state;
            int         off;    /* 0-based index of this subscript */
            bool        isupper;    /* is it upper or lower subscript? */
            int         jumpdone;   /* jump here on null */
        }           arrayref_subscript;

/* for EEOP_ARRAYREF_OLD / ASSIGN / FETCH */
        struct
        {
            /* too big to have inline */
            struct ArrayRefState *state;
        }           arrayref;

/* for EEOP_DOMAIN_NOTNULL / DOMAIN_CHECK */
        struct
        {
            /* name of constraint */
            char       *constraintname;
            /* where the result of a CHECK constraint will be stored */
            Datum      *checkvalue;
            bool       *checknull;
            /* OID of domain type */
            Oid         resulttype;
        }           domaincheck;

/* for EEOP_CONVERT_ROWTYPE */
        struct
        {
            ConvertRowtypeExpr *convert;    /* original expression */
            /* these three fields are filled at runtime: */
            TupleDesc   indesc; /* tupdesc for input type */
            TupleDesc   outdesc;    /* tupdesc for output type */
            TupleConversionMap *map;    /* column mapping */
            bool        initialized;    /* initialized for current types? */
        }           convert_rowtype;

/* for EEOP_SCALARARRAYOP */
        struct
        {
            /* element_type/typlen/typbyval/typalign are filled at runtime */
            Oid         element_type;   /* InvalidOid if not yet filled */
            bool        useOr;  /* use OR or AND semantics? */
            int16       typlen; /* array element type storage info */
            bool        typbyval;
            char        typalign;
            FmgrInfo   *finfo;  /* function's lookup data */
            FunctionCallInfo fcinfo_data;   /* arguments etc */
            /* faster to access without additional indirection: */
            PGFunction  fn_addr;    /* actual call address */
        }           scalararrayop;

/* for EEOP_XMLEXPR */
        struct
        {
            XmlExpr    *xexpr;  /* original expression node */
            /* workspace for evaluating named args, if any */
            Datum      *named_argvalue;
            bool       *named_argnull;
            /* workspace for evaluating unnamed args, if any */
            Datum      *argvalue;
            bool       *argnull;
        }           xmlexpr;

/* for EEOP_AGGREF */
        struct
        {
            /* out-of-line state, modified by nodeAgg.c */
            AggrefExprState *astate;
        }           aggref;

/* for EEOP_GROUPING_FUNC */
        struct
        {
            AggState   *parent; /* parent Agg */
            List       *clauses;    /* integer list of column numbers */
        }           grouping_func;

/* for EEOP_WINDOW_FUNC */
        struct
        {
            /* out-of-line state, modified by nodeWindowFunc.c */
            WindowFuncExprState *wfstate;
        }           window_func;

/* for EEOP_SUBPLAN */
        struct
        {
            /* out-of-line state, created by nodeSubplan.c */
            SubPlanState *sstate;
        }           subplan;

/* for EEOP_ALTERNATIVE_SUBPLAN */
        struct
        {
            /* out-of-line state, created by nodeSubplan.c */
            AlternativeSubPlanState *asstate;
        }           alternative_subplan;

/* for EEOP_AGG_*DESERIALIZE */
        struct
        {
            AggState   *aggstate;
            FunctionCallInfo fcinfo_data;
            int         jumpnull;
        }           agg_deserialize;

/* for EEOP_AGG_STRICT_INPUT_CHECK */
        struct
        {
            bool       *nulls;
            int         nargs;
            int         jumpnull;
        }           agg_strict_input_check;

/* for EEOP_AGG_INIT_TRANS */
        struct
        {
            AggState   *aggstate;
            AggStatePerTrans pertrans;
            ExprContext *aggcontext;
            int         setno;
            int         transno;
            int         setoff;
            int         jumpnull;
        }           agg_init_trans;

/* for EEOP_AGG_STRICT_TRANS_CHECK */
        struct
        {
            AggState   *aggstate;
            int         setno;
            int         transno;
            int         setoff;
            int         jumpnull;
        }           agg_strict_trans_check;

/* for EEOP_AGG_{PLAIN,ORDERED}_TRANS* */
        struct
        {
            AggState   *aggstate;
            AggStatePerTrans pertrans;
            ExprContext *aggcontext;
            int         setno;
            int         transno;
            int         setoff;
        }           agg_trans;
    }           d;
} ExprEvalStep;

```

**ExprEvalOp**
ExprEvalSteps的鑑頻器,定義哪個操作將被執行並且聯合體ExprEvalStep->d中的哪個struct將被使用.
```
/*
 * Discriminator for ExprEvalSteps.
 * ExprEvalSteps的鑑頻器
 *
 * Identifies the operation to be executed and which member in the
 * ExprEvalStep->d union is valid.
 * 定義哪個操作將被執行並且聯合體ExprEvalStep->d中的哪個struct將被使用.
 *
 * The order of entries needs to be kept in sync with the dispatch_table[]
 * array in execExprInterp.c:ExecInterpExpr().
 * 條目的排序需要與execExprInterp.c:ExecInterpExpr()中dispatch_table[]陣列的元素保持一致
 */
typedef enum ExprEvalOp
{
    /* entire expression has been evaluated completely, return */
    //整個表示式已被解析,返回
    EEOP_DONE,

/* apply slot_getsomeattrs on corresponding tuple slot */
    //在相應的元組slot上應用了slot_getsomeattrs方法
    EEOP_INNER_FETCHSOME,
    EEOP_OUTER_FETCHSOME,
    EEOP_SCAN_FETCHSOME,

/* compute non-system Var value */
    //計算非系統Var變數值
    EEOP_INNER_VAR,
    EEOP_OUTER_VAR,
    EEOP_SCAN_VAR,

/* compute system Var value */
    //計算系統Var變數值
    EEOP_INNER_SYSVAR,
    EEOP_OUTER_SYSVAR,
    EEOP_SCAN_SYSVAR,

/* compute wholerow Var */
    //計算整行Var
    EEOP_WHOLEROW,

/*
     * Compute non-system Var value, assign it into ExprState's resultslot.
     * These are not used if a CheckVarSlotCompatibility() check would be
     * needed.
     * 計算非系統Var值,分配到ExprState's的resultslot欄位中.
     * 如果CheckVarSlotCompatibility()需要時,這些都不需要.
     */
    EEOP_ASSIGN_INNER_VAR,
    EEOP_ASSIGN_OUTER_VAR,
    EEOP_ASSIGN_SCAN_VAR,

/* assign ExprState's resvalue/resnull to a column of its resultslot */
    //分配ExprState's resvalue/resnull到該列的resultslot中
    EEOP_ASSIGN_TMP,
    /* ditto, applying MakeExpandedObjectReadOnly() */
    //同上,應用MakeExpandedObjectReadOnly()
    EEOP_ASSIGN_TMP_MAKE_RO,

/* evaluate Const value */
    //解析常量值
    EEOP_CONST,

/*
     * Evaluate function call (including OpExprs etc).  For speed, we
     * distinguish in the opcode whether the function is strict and/or
     * requires usage stats tracking.
     * 解析函式呼叫(包括OpExprs等等).
     * 出於效能的考慮,需要區分opcode是strict函式還是非strict函式,以及是否需要統計跟蹤.
     */
    EEOP_FUNCEXPR,
    EEOP_FUNCEXPR_STRICT,
    EEOP_FUNCEXPR_FUSAGE,
    EEOP_FUNCEXPR_STRICT_FUSAGE,

/*
     * Evaluate boolean AND expression, one step per subexpression. FIRST/LAST
     * subexpressions are special-cased for performance.  Since AND always has
     * at least two subexpressions, FIRST and LAST never apply to the same
     * subexpression.
     * 解析布林AND表示式,每一個子表示式一個步驟.
     * FIRST/LAST子表示式是效能上的特例.
     * 由於AND通常至少有兩個子表示式,FIRST和LAST永遠都不會應用在同一個子表示式上.
     */
    EEOP_BOOL_AND_STEP_FIRST,
    EEOP_BOOL_AND_STEP,
    EEOP_BOOL_AND_STEP_LAST,

/* similarly for boolean OR expression */
    //與布林OR表示式類似
    EEOP_BOOL_OR_STEP_FIRST,
    EEOP_BOOL_OR_STEP,
    EEOP_BOOL_OR_STEP_LAST,

/* evaluate boolean NOT expression */
    //解析布林NOT表示式
    EEOP_BOOL_NOT_STEP,

/* simplified version of BOOL_AND_STEP for use by ExecQual() */
    //用於ExecQual()中的BOOL_AND_STEP簡化版本
    EEOP_QUAL,

/* unconditional jump to another step */
    //無條件跳轉到另外一個步驟
    EEOP_JUMP,

/* conditional jumps based on current result value */
    //基於當前結果值的條件跳轉
    EEOP_JUMP_IF_NULL,
    EEOP_JUMP_IF_NOT_NULL,
    EEOP_JUMP_IF_NOT_TRUE,

/* perform NULL tests for scalar values */
    //為scalar值執行NULL測試
    EEOP_NULLTEST_ISNULL,
    EEOP_NULLTEST_ISNOTNULL,

/* perform NULL tests for row values */
    //為行值執行NULL測試
    EEOP_NULLTEST_ROWISNULL,
    EEOP_NULLTEST_ROWISNOTNULL,

/* evaluate a BooleanTest expression */
    //解析BooleanTest表示式
    EEOP_BOOLTEST_IS_TRUE,
    EEOP_BOOLTEST_IS_NOT_TRUE,
    EEOP_BOOLTEST_IS_FALSE,
    EEOP_BOOLTEST_IS_NOT_FALSE,

/* evaluate PARAM_EXEC/EXTERN parameters */
    //解析PARAM_EXEC/EXTERN引數
    EEOP_PARAM_EXEC,
    EEOP_PARAM_EXTERN,
    EEOP_PARAM_CALLBACK,

/* return CaseTestExpr value */
    //返回CaseTestExpr值
    EEOP_CASE_TESTVAL,

/* apply MakeExpandedObjectReadOnly() to target value */
    //對目標值應用MakeExpandedObjectReadOnly()
    EEOP_MAKE_READONLY,

/* evaluate assorted special-purpose expression types */
    //解析各種特殊用途的表示式型別
    EEOP_IOCOERCE,
    EEOP_DISTINCT,
    EEOP_NOT_DISTINCT,
    EEOP_NULLIF,
    EEOP_SQLVALUEFUNCTION,
    EEOP_CURRENTOFEXPR,
    EEOP_NEXTVALUEEXPR,
    EEOP_ARRAYEXPR,
    EEOP_ARRAYCOERCE,
    EEOP_ROW,

/*
     * Compare two individual elements of each of two compared ROW()
     * expressions.  Skip to ROWCOMPARE_FINAL if elements are not equal.
     * 給出兩個需要對比的ROW()表示式,兩兩比較行中的元素.
     * 如果元素不相等,則跳轉到ROWCOMPARE_FINAL
     */
    EEOP_ROWCOMPARE_STEP,

/* evaluate boolean value based on previous ROWCOMPARE_STEP operations */
    //基於上一步的ROWCOMPARE_STEP操作解析布林值
    EEOP_ROWCOMPARE_FINAL,

/* evaluate GREATEST() or LEAST() */
    //解析GREATEST()和LEAST()
    EEOP_MINMAX,

/* evaluate FieldSelect expression */
    //解析FieldSelect表示式
    EEOP_FIELDSELECT,

/*
     * Deform tuple before evaluating new values for individual fields in a
     * FieldStore expression.
     * 在解析FieldStore表示式中的獨立列新值前重構元組
     */
    EEOP_FIELDSTORE_DEFORM,

/*
     * Form the new tuple for a FieldStore expression.  Individual fields will
     * have been evaluated into columns of the tuple deformed by the preceding
     * DEFORM step.
     * 為FieldStore表示式構成新元組.
     * 單獨的欄位會解析到元組的列中(行已被上一個步驟EEOP_FIELDSTORE_DEFORM析構)
     */
    EEOP_FIELDSTORE_FORM,

/* Process an array subscript; short-circuit expression to NULL if NULL */
    //處理陣列子指令碼.如為NULL則短路表示式為NULL
    EEOP_ARRAYREF_SUBSCRIPT,

/*
     * Compute old array element/slice when an ArrayRef assignment expression
     * contains ArrayRef/FieldStore subexpressions.  Value is accessed using
     * the CaseTest mechanism.
     * 在ArrayRef分配表示式包含ArrayRef/FieldStore子表示式時計算舊的陣列元素/片.
     * 透過CaseTest機制訪問Value
     */
    EEOP_ARRAYREF_OLD,

/* compute new value for ArrayRef assignment expression */
    //為ArrayRef分配118
    EEOP_ARRAYREF_ASSIGN,

/* compute element/slice for ArrayRef fetch expression */
    //為ArrayRef提取表示式計算element/slice
    EEOP_ARRAYREF_FETCH,

/* evaluate value for CoerceToDomainValue */
    //為CoerceToDomainValue解析值
    EEOP_DOMAIN_TESTVAL,

/* evaluate a domain's NOT NULL constraint */
    //解析域 NOT NULL 約束
    EEOP_DOMAIN_NOTNULL,

/* evaluate a single domain CHECK constraint */
    //解析單個域CHECK約束
    EEOP_DOMAIN_CHECK,

/* evaluate assorted special-purpose expression types */
    //解析特殊目的的表示式型別
    EEOP_CONVERT_ROWTYPE,
    EEOP_SCALARARRAYOP,
    EEOP_XMLEXPR,
    EEOP_AGGREF,
    EEOP_GROUPING_FUNC,
    EEOP_WINDOW_FUNC,
    EEOP_SUBPLAN,
    EEOP_ALTERNATIVE_SUBPLAN,

/* aggregation related nodes */
    //聚合相關節點
    EEOP_AGG_STRICT_DESERIALIZE,
    EEOP_AGG_DESERIALIZE,
    EEOP_AGG_STRICT_INPUT_CHECK,
    EEOP_AGG_INIT_TRANS,
    EEOP_AGG_STRICT_TRANS_CHECK,
    EEOP_AGG_PLAIN_TRANS_BYVAL,
    EEOP_AGG_PLAIN_TRANS,
    EEOP_AGG_ORDERED_TRANS_DATUM,
    EEOP_AGG_ORDERED_TRANS_TUPLE,

/* non-existent operation, used e.g. to check array lengths */
    //不存在的操作,比如用於檢測陣列長度
    EEOP_LAST
} ExprEvalOp;

```

### 二、原始碼解讀
**ExecInitIndexScan**
初始化Index Scan執行期狀態資訊,呼叫ExecAssignScanProjectionInfo->...->ExecBuildProjectionInfo函式構建投影資訊.
```
/* ----------------------------------------------------------------
 *      ExecInitIndexScan
 *
 *      Initializes the index scan's state information, creates
 *      scan keys, and opens the base and index relations.
 *
 *      Note: index scans have 2 sets of state information because
 *            we have to keep track of the base relation and the
 *            index relation.
 * ----------------------------------------------------------------
 */
IndexScanState *
ExecInitIndexScan(IndexScan *node, EState *estate, int eflags)
{
    ...
     /*
     * Initialize result slot, type and projection.
     * 初始化結果slot,型別和投影
     */
    ExecInitResultTupleSlotTL(estate, &indexstate->ss.ps);
    ExecAssignScanProjectionInfo(&indexstate->ss);
    ...
}

```

**ExecBuildProjectionInfo**
為給定的econtext中的tlist構建ProjectionInfo,並把結果儲存在tuple slot中.(呼叫者必須確保元組slot與此tlist匹配)
其主要邏輯是:
1.初始化
2.如需要,插入EEOP_*_FETCHSOME步驟(呼叫ExecInitExprSlots)
3.遍歷targetList,處理targetList(目標列)中的每一個列
3.1.對於"安全"Var則只需要生成EEOP_ASSIGN_*_VAR步驟
3.2對於非"安全"VAr,使用常規辦法處理列表示式,呼叫ExecInitExprRec函式處理,並透過ExprEvalPushStep壓步驟
4.壓入EEOP_DONE步驟

```
/*
 * ExecAssignScanProjectionInfo
 *      Set up projection info for a scan node, if necessary.
 * ExecAssignScanProjectionInfo
 *      為掃描節點配置投影資訊.
 *
 * We can avoid a projection step if the requested tlist exactly matches
 * the underlying tuple type.  If so, we just set ps_ProjInfo to NULL.
 * Note that this case occurs not only for simple "SELECT * FROM ...", but
 * also in most cases where there are joins or other processing nodes above
 * the scan node, because the planner will preferentially generate a matching
 * tlist.
 * 如果請求的tlist剛好與潛在的tuple型別相匹配,則可以避免投影這一步驟.
 * 因此,只需要簡單的把ps_ProjInfo設定為NULL即可.
 * 注意這種情況不單在"SELECT * FROM ..."時會出現,
 *   而且在存在連線或者其他處理節點在掃描節點的上層時也會發生,
 * 因為規劃器會優先生成匹配的tlist.
 *
 * The scan slot's descriptor must have been set already.
 * 掃描slot的描述符必須已設定.
 */
void
ExecAssignScanProjectionInfo(ScanState *node)
{
    //掃描節點
    Scan       *scan = (Scan *) node->ps.plan;
    //元組描述符
    TupleDesc   tupdesc = node->ss_ScanTupleSlot->tts_tupleDescriptor;
    //執行ExecConditionalAssignProjectionInfo
    ExecConditionalAssignProjectionInfo(&node->ps, tupdesc, scan->scanrelid);
}

/* ----------------
 *      ExecConditionalAssignProjectionInfo
 *
 * as ExecAssignProjectionInfo, but store NULL rather than building projection
 * info if no projection is required
 * 與ExecAssignProjectionInfo類似,但在不需要投影操作時只需要儲存NULL而不是構建投影資訊
 * ----------------
 */
void
ExecConditionalAssignProjectionInfo(PlanState *planstate, TupleDesc inputDesc,
                                    Index varno)
{
    //正好匹配元組型別,則設計為NULL
    if (tlist_matches_tupdesc(planstate,
                              planstate->plan->targetlist,
                              varno,
                              inputDesc))
        planstate->ps_ProjInfo = NULL;
    else
        //否則執行ExecAssignProjectionInfo
        ExecAssignProjectionInfo(planstate, inputDesc);
}

/* ----------------
 *      ExecAssignProjectionInfo
 *
 * forms the projection information from the node's targetlist
 * 透過節點的targetlist構造投影資訊.
 *
 * Notes for inputDesc are same as for ExecBuildProjectionInfo: supply it
 * for a relation-scan node, can pass NULL for upper-level nodes
 * 注意inputDesc與ExecBuildProjectionInfo的一樣:
 *   為relation-scan節點提供該描述符,可能為高層的節點傳遞NULL值
 * ----------------
 */
void
ExecAssignProjectionInfo(PlanState *planstate,
                         TupleDesc inputDesc)
{
    //直接呼叫ExecBuildProjectionInfo
    planstate->ps_ProjInfo =
        ExecBuildProjectionInfo(planstate->plan->targetlist,
                                planstate->ps_ExprContext,
                                planstate->ps_ResultTupleSlot,
                                planstate,
                                inputDesc);
}

/*
 *      ExecBuildProjectionInfo
 *
 * Build a ProjectionInfo node for evaluating the given tlist in the given
 * econtext, and storing the result into the tuple slot.  (Caller must have
 * ensured that tuple slot has a descriptor matching the tlist!)
 * 為給定的econtext中的tlist構建ProjectionInfo,並把結果儲存在tuple slot中.
 * (呼叫者必須確保元組slot與此tlist匹配)
 *
 * inputDesc can be NULL, but if it is not, we check to see whether simple
 * Vars in the tlist match the descriptor.  It is important to provide
 * inputDesc for relation-scan plan nodes, as a cross check that the relation
 * hasn't been changed since the plan was made.  At higher levels of a plan,
 * there is no need to recheck.
 * inputDesc可以為NULL,但如果不是,檢查tlist中的簡單Vars是否與描述符匹配.
 * 對於relation-scan節點來說,提供inputDesc是十分重要的,
 *   交叉檢查在plan已生成的情況下relation沒有出現變化.
 * 而plan更高的層次,則不需要重新檢查.
 *
 * This is implemented by internally building an ExprState that performs the
 * whole projection in one go.
 * 透過內部構造ExprState(在一輪執行中完成整個投影操作)來實現.
 *
 * Caution: before PG v10, the targetList was a list of ExprStates; now it
 * should be the planner-created targetlist, since we do the compilation here.
 * 注意:在PG v10前,targetlist是ExprState連結串列,現在tlist應該是planner建立的targetlist,
 *   如不是則需報錯.
 */
ProjectionInfo *
ExecBuildProjectionInfo(List *targetList,
                        ExprContext *econtext,
                        TupleTableSlot *slot,
                        PlanState *parent,
                        TupleDesc inputDesc)
{
    //生成ProjectionInfo節點
    ProjectionInfo *projInfo = makeNode(ProjectionInfo);
    //表示式狀態節點
    ExprState  *state;
    //表示式解析步驟
    ExprEvalStep scratch = {0};
    //臨時變數
    ListCell   *lc;
    //expr上下文
    projInfo->pi_exprContext = econtext;
    /* We embed ExprState into ProjectionInfo instead of doing extra palloc */
    //整合ExprState到ProjectionInfo中,而不需要另外的palloc
    projInfo->pi_state.tag.type = T_ExprState;
    state = &projInfo->pi_state;
    state->expr = (Expr *) targetList;
    state->parent = parent;
    state->ext_params = NULL;

state->resultslot = slot;

/* Insert EEOP_*_FETCHSOME steps as needed */
    //如需要,插入EEOP_*_FETCHSOME步驟
    ExecInitExprSlots(state, (Node *) targetList);

/* Now compile each tlist column */
    //現在"編譯"tlist中的每一個列
    foreach(lc, targetList)
    {
        TargetEntry *tle = lfirst_node(TargetEntry, lc);
        Var        *variable = NULL;
        AttrNumber  attnum = 0;
        bool        isSafeVar = false;

/*
         * If tlist expression is a safe non-system Var, use the fast-path
         * ASSIGN_*_VAR opcodes.  "Safe" means that we don't need to apply
         * CheckVarSlotCompatibility() during plan startup.  If a source slot
         * was provided, we make the equivalent tests here; if a slot was not
         * provided, we assume that no check is needed because we're dealing
         * with a non-relation-scan-level expression.
         * 如果tlist表示式是安全的非系統Var,使用快速路徑ASSIGN_*_VAR opcodes.
         * "Safe"的意思是在計劃啟動執行時我們不需要應用CheckVarSlotCompatibility().
         * 如果提供了源slot,假定不需要再做檢查,因為我們正在處理的是非關係掃描級別的表示式.
         */
        if (tle->expr != NULL &&
            IsA(tle->expr, Var) &&
            ((Var *) tle->expr)->varattno > 0)
        {
            /* Non-system Var, but how safe is it? */
            //非系統Var,但有多安全呢?
            variable = (Var *) tle->expr;
            attnum = variable->varattno;

if (inputDesc == NULL)
                //無法檢查,假定是OK的.
                isSafeVar = true;   /* can't check, just assume OK */
            else if (attnum <= inputDesc->natts)
            {
                //---- 屬性編號小於輸入的屬性個數
                Form_pg_attribute attr = TupleDescAttr(inputDesc, attnum - 1);

/*
                 * If user attribute is dropped or has a type mismatch, don't
                 * use ASSIGN_*_VAR.  Instead let the normal expression
                 * machinery handle it (which'll possibly error out).
                 * 如果使用者屬性已被清除或者有型別不匹配,不要使用ASSIGN_*_VAR.
                 * 讓常規的表示式匹配處理這周情況(可能會出現錯誤)
                 */
                if (!attr->attisdropped && variable->vartype == attr->atttypid)
                {
                    isSafeVar = true;
                }
            }
        }

if (isSafeVar)
        {
            //--- 如為安全的Var
            /* Fast-path: just generate an EEOP_ASSIGN_*_VAR step */
            //Fast-path: 只需要生成EEOP_ASSIGN_*_VAR步驟即可
            switch (variable->varno)
            {
                case INNER_VAR:
                    /* get the tuple from the inner node */
                    //內關係VAR,從inner節點獲取元組
                    scratch.opcode = EEOP_ASSIGN_INNER_VAR;
                    break;

case OUTER_VAR:
                    /* get the tuple from the outer node */
                    //從外關係獲取元組
                    scratch.opcode = EEOP_ASSIGN_OUTER_VAR;
                    break;

/* INDEX_VAR is handled by default case */
                    //預設: INDEX_VAR
                default:
                    /* get the tuple from the relation being scanned */
                    //從正在掃描的關係中獲取元組
                    scratch.opcode = EEOP_ASSIGN_SCAN_VAR;
                    break;
            }
            //EEOP_ASSIGN_*_VAR
            scratch.d.assign_var.attnum = attnum - 1;
            scratch.d.assign_var.resultnum = tle->resno - 1;
            ExprEvalPushStep(state, &scratch);
        }
        else
        {
            /*
             * Otherwise, compile the column expression normally.
             * 否則的話,使用常規辦法"編譯"列表示式
             *
             * We can't tell the expression to evaluate directly into the
             * result slot, as the result slot (and the exprstate for that
             * matter) can change between executions.  We instead evaluate
             * into the ExprState's resvalue/resnull and then move.
             * 我們不能夠直接告知表示式進行解析到結果slot中,
             *   因為結果slot(以及這種情況下的exprstate)在執行期間會出現變化.
             * 我們只需要解析到ExprState's的resvalue/resnull中並進行移動即可.
             */
            ExecInitExprRec(tle->expr, state,
                            &state->resvalue, &state->resnull);

/*
             * Column might be referenced multiple times in upper nodes, so
             * force value to R/O - but only if it could be an expanded datum.
             * 列可能在高層節點被依賴多次,因此強制值為R/O - 只在可擴充套件datum時才這樣處理
             */
            //d.assign_tmp.resultnum/attnum
            if (get_typlen(exprType((Node *) tle->expr)) == -1)
                scratch.opcode = EEOP_ASSIGN_TMP_MAKE_RO;
            else
                scratch.opcode = EEOP_ASSIGN_TMP;
            scratch.d.assign_tmp.resultnum = tle->resno - 1;
            ExprEvalPushStep(state, &scratch);
        }
    }

scratch.opcode = EEOP_DONE;
    ExprEvalPushStep(state, &scratch);

ExecReadyExpr(state);
    //返回投影資訊.
    return projInfo;
}

```

### 三、跟蹤分析
測試指令碼
```
testdb=# select 1+id,c2 from t_expr where id < 3;

```
呼叫棧
```
(gdb) bt
#0  ExecBuildProjectionInfo (targetList=0x1cc7550, econtext=0x1c8f408, slot=0x1c8f710, parent=0x1c8f1f0, 
    inputDesc=0x7f8386bb6ab8) at execExpr.c:355
#1  0x00000000006e60d5 in ExecAssignProjectionInfo (planstate=0x1c8f1f0, inputDesc=0x7f8386bb6ab8) at execUtils.c:468
#2  0x00000000006e613c in ExecConditionalAssignProjectionInfo (planstate=0x1c8f1f0, inputDesc=0x7f8386bb6ab8, varno=1)
    at execUtils.c:493
#3  0x00000000006e23f5 in ExecAssignScanProjectionInfo (node=0x1c8f1f0) at execScan.c:240
#4  0x0000000000700afc in ExecInitIndexScan (node=0x1ba8a18, estate=0x1c8efd8, eflags=16) at nodeIndexscan.c:962
#5  0x00000000006e00cc in ExecInitNode (node=0x1ba8a18, estate=0x1c8efd8, eflags=16) at execProcnode.c:217
#6  0x00000000006d6abe in InitPlan (queryDesc=0x1c94aa8, eflags=16) at execMain.c:1046
#7  0x00000000006d58ad in standard_ExecutorStart (queryDesc=0x1c94aa8, eflags=16) at execMain.c:265
#8  0x00000000006d5649 in ExecutorStart (queryDesc=0x1c94aa8, eflags=0) at execMain.c:147
#9  0x00000000008c18d6 in PortalStart (portal=0x1c15608, params=0x0, eflags=0, snapshot=0x0) at pquery.c:520
#10 0x00000000008bbe1b in exec_simple_query (query_string=0x1ba6d78 "select 1+id,c2 from t_expr where id < 3;")
    at postgres.c:1106
#11 0x00000000008c0191 in PostgresMain (argc=1, argv=0x1bd4cb8, dbname=0x1bd4b20 "testdb", username=0x1ba3a98 "xdb")
    at postgres.c:4182
#12 0x000000000081e06c in BackendRun (port=0x1bc8ae0) at postmaster.c:4361
#13 0x000000000081d7df in BackendStartup (port=0x1bc8ae0) at postmaster.c:4033
#14 0x0000000000819bd9 in ServerLoop () at postmaster.c:1706
#15 0x000000000081948f in PostmasterMain (argc=1, argv=0x1ba1a50) at postmaster.c:1379
#16 0x0000000000742931 in main (argc=1, argv=0x1ba1a50) at main.c:228
```
執行跟蹤,進入函式ExecBuildProjectionInfo
```
(gdb) b ExecBuildProjectionInfo
Breakpoint 1 at 0x6c5377: file execExpr.c, line 355.
(gdb) c
Continuing.

Breakpoint 1, ExecBuildProjectionInfo (targetList=0x1c93498, econtext=0x1c883a8, slot=0x1c887c8, parent=0x1c88190, 
    inputDesc=0x1c884a0) at execExpr.c:355
355     ProjectionInfo *projInfo = makeNode(ProjectionInfo);
(gdb) 
```
1.初始化
```
(gdb) n
357     ExprEvalStep scratch = {0};
(gdb) 
360     projInfo->pi_exprContext = econtext;
(gdb) 
362     projInfo->pi_state.tag.type = T_ExprState;
(gdb) 
363     state = &projInfo->pi_state;
(gdb) 
364     state->expr = (Expr *) targetList;
(gdb) 
365     state->parent = parent;
(gdb) 
366     state->ext_params = NULL;
(gdb) 
368     state->resultslot = slot;
(gdb) 
```
檢視相關變數
```
(gdb) p *state
$1 = {tag = {type = T_ExprState}, flags = 0 '\000', resnull = false, resvalue = 0, resultslot = 0x1c8f710, steps = 0x0, 
  evalfunc = 0x0, expr = 0x1cc7550, evalfunc_private = 0x0, steps_len = 0, steps_alloc = 0, parent = 0x1c8f1f0, 
  ext_params = 0x0, innermost_caseval = 0x0, innermost_casenull = 0x0, innermost_domainval = 0x0, 
  innermost_domainnull = 0x0}
```
目標列連結串列
```  
(gdb) p targetList
$2 = (List *) 0x1cc7550
(gdb) p *targetList
$3 = {type = T_List, length = 2, head = 0x1cc7528, tail = 0x1cc75e0}
```
第一個元素是1+id,第二個元素是c2
```
(gdb) p *(TargetEntry *)targetList->head->data.ptr_value
$7 = {xpr = {type = T_TargetEntry}, expr = 0x1c9a930, resno = 1, resname = 0xbcf498 "?column?", ressortgroupref = 0, 
  resorigtbl = 0, resorigcol = 0, resjunk = false}
(gdb) p *(OpExpr *)((TargetEntry *)targetList->head->data.ptr_value)->expr
$9 = {xpr = {type = T_OpExpr}, opno = 551, opfuncid = 177, opresulttype = 23, opretset = false, opcollid = 0, 
  inputcollid = 0, args = 0x1c9a878, location = 8}
(gdb) p *(Node *)targetList->head->next->data.ptr_value
$10 = {type = T_TargetEntry}
(gdb) p *(TargetEntry *)targetList->head->next->data.ptr_value
$11 = {xpr = {type = T_TargetEntry}, expr = 0x1c9aa40, resno = 2, resname = 0x1ba7a40 "c2", ressortgroupref = 0, 
  resorigtbl = 237600, resorigcol = 2, resjunk = false}
```
2.如需要,插入EEOP_*_FETCHSOME步驟(呼叫ExecInitExprSlots)
```
(gdb) 
371     ExecInitExprSlots(state, (Node *) targetList);
```
第一個步驟,opcode = 3,即EEOP_SCAN_FETCHSOME
```
(gdb) n
374     foreach(lc, targetList)
(gdb) p *state
$13 = {tag = {type = T_ExprState}, flags = 0 '\000', resnull = false, resvalue = 0, resultslot = 0x1c8f710, 
  steps = 0x1c8f868, evalfunc = 0x0, expr = 0x1cc7550, evalfunc_private = 0x0, steps_len = 1, steps_alloc = 16, 
  parent = 0x1c8f1f0, ext_params = 0x0, innermost_caseval = 0x0, innermost_casenull = 0x0, innermost_domainval = 0x0, 
  innermost_domainnull = 0x0}
(gdb) p state->steps[0]
$14 = {opcode = 3, resvalue = 0x0, resnull = 0x0, d = {fetch = {last_var = 2, known_desc = 0x0}, var = {attnum = 2, 
      vartype = 0}, wholerow = {var = 0x2, first = false, slow = false, tupdesc = 0x0, junkFilter = 0x0}, assign_var = {
      resultnum = 2, attnum = 0}, assign_tmp = {resultnum = 2}, constval = {value = 2, isnull = false}, func = {
      finfo = 0x2, fcinfo_data = 0x0, fn_addr = 0x0, nargs = 0}, boolexpr = {anynull = 0x2, jumpdone = 0}, qualexpr = {
      jumpdone = 2}, jump = {jumpdone = 2}, nulltest_row = {argdesc = 0x2}, param = {paramid = 2, paramtype = 0}, cparam = {
      paramfunc = 0x2, paramarg = 0x0, paramid = 0, paramtype = 0}, casetest = {value = 0x2, isnull = 0x0}, 
    make_readonly = {value = 0x2, isnull = 0x0}, iocoerce = {finfo_out = 0x2, fcinfo_data_out = 0x0, finfo_in = 0x0, 
      fcinfo_data_in = 0x0}, sqlvaluefunction = {svf = 0x2}, nextvalueexpr = {seqid = 2, seqtypid = 0}, arrayexpr = {
      elemvalues = 0x2, elemnulls = 0x0, nelems = 0, elemtype = 0, elemlength = 0, elembyval = false, elemalign = 0 '\000', 
      multidims = false}, arraycoerce = {elemexprstate = 0x2, resultelemtype = 0, amstate = 0x0}, row = {tupdesc = 0x2, 
      elemvalues = 0x0, elemnulls = 0x0}, rowcompare_step = {finfo = 0x2, fcinfo_data = 0x0, fn_addr = 0x0, jumpnull = 0, 
      jumpdone = 0}, rowcompare_final = {rctype = ROWCOMPARE_LE}, minmax = {values = 0x2, nulls = 0x0, nelems = 0, 
      op = IS_GREATEST, finfo = 0x0, fcinfo_data = 0x0}, fieldselect = {fieldnum = 2, resulttype = 0, argdesc = 0x0}, 
    fieldstore = {fstore = 0x2, argdesc = 0x0, values = 0x0, nulls = 0x0, ncolumns = 0}, arrayref_subscript = {state = 0x2, 
      off = 0, isupper = false, jumpdone = 0}, arrayref = {state = 0x2}, domaincheck = {
      constraintname = 0x2 <address out="" of="" bounds="">, checkvalue = 0x0, checknull = 0x0, resulttype = 0}, 
    convert_rowtype = {convert = 0x2, indesc = 0x0, outdesc = 0x0, map = 0x0, initialized = false}, scalararrayop = {
      element_type = 2, useOr = false, typlen = 0, typbyval = false, typalign = 0 '\000', finfo = 0x0, fcinfo_data = 0x0, 
      fn_addr = 0x0}, xmlexpr = {xexpr = 0x2, named_argvalue = 0x0, named_argnull = 0x0, argvalue = 0x0, argnull = 0x0}, 
    aggref = {astate = 0x2}, grouping_func = {parent = 0x2, clauses = 0x0}, window_func = {wfstate = 0x2}, subplan = {
      sstate = 0x2}, alternative_subplan = {asstate = 0x2}, agg_deserialize = {aggstate = 0x2, fcinfo_data = 0x0, 
      jumpnull = 0}, agg_strict_input_check = {nulls = 0x2, nargs = 0, jumpnull = 0}, agg_init_trans = {aggstate = 0x2, 
      pertrans = 0x0, aggcontext = 0x0, setno = 0, transno = 0, setoff = 0, jumpnull = 0}, agg_strict_trans_check = {
      aggstate = 0x2, setno = 0, transno = 0, setoff = 0, jumpnull = 0}, agg_trans = {aggstate = 0x2, pertrans = 0x0, 
      aggcontext = 0x0, setno = 0, transno = 0, setoff = 0}}}
(gdb) 
```
3.遍歷targetList,處理targetList(目標列)中的每一個列
3.1.對於"安全"Var則只需要生成EEOP_ASSIGN_*_VAR步驟
3.2.對於非"安全"VAr,使用常規辦法處理列表示式,呼叫ExecInitExprRec函式處理,並透過ExprEvalPushStep壓步驟
```
(gdb) n
376         TargetEntry *tle = lfirst_node(TargetEntry, lc);
(gdb) 
377         Var        *variable = NULL;
(gdb) 
378         AttrNumber  attnum = 0;
(gdb) 
379         bool        isSafeVar = false;
(gdb) 
389         if (tle->expr != NULL &&
(gdb) 
390             IsA(tle->expr, Var) &&
(gdb) 
389         if (tle->expr != NULL &&
(gdb) 
415         if (isSafeVar)
(gdb) p *tle
$15 = {xpr = {type = T_TargetEntry}, expr = 0x1c9a930, resno = 1, resname = 0xbcf498 "?column?", ressortgroupref = 0, 
  resorigtbl = 0, resorigcol = 0, resjunk = false}
(gdb) n
452             ExecInitExprRec(tle->expr, state,
(gdb) 
```
進入ExecInitExprRec,Node節點為OpExpr,執行ExprEvalPushStep壓入步驟中
```
(gdb) step
ExecInitExprRec (node=0x1c9a930, state=0x1c8f7d8, resv=0x1c8f7e0, resnull=0x1c8f7dd) at execExpr.c:645
645     ExprEvalStep scratch = {0};
(gdb) n
648     check_stack_depth();
(gdb) 
651     Assert(resv != NULL && resnull != NULL);
(gdb) 
652     scratch.resvalue = resv;
(gdb) 
653     scratch.resnull = resnull;
(gdb) 
656     switch (nodeTag(node))
(gdb) 
891                 OpExpr     *op = (OpExpr *) node;
(gdb) p *node
$16 = {type = T_OpExpr}
(gdb) n
893                 ExecInitFunc(&scratch, node,
(gdb) 
896                 ExprEvalPushStep(state, &scratch);
(gdb) 
897                 break;
(gdb) 
2122    }
(gdb) 
ExecBuildProjectionInfo (targetList=0x1cc7550, econtext=0x1c8f408, slot=0x1c8f710, parent=0x1c8f1f0, 
    inputDesc=0x7f8386bb6ab8) at execExpr.c:459
459             if (get_typlen(exprType((Node *) tle->expr)) == -1)
(gdb) 
462                 scratch.opcode = EEOP_ASSIGN_TMP;
(gdb) 
```
ExecInitExprRec呼叫完畢,增加了2個步驟,分別是:
1.opcode = 6,即EEOP_SCAN_VAR
2.opcode = 18,即EEOP_FUNCEXPR_STRICT
```
(gdb) p *state
$17 = {tag = {type = T_ExprState}, flags = 0 '\000', resnull = false, resvalue = 0, resultslot = 0x1c8f710, 
  steps = 0x1c8f868, evalfunc = 0x0, expr = 0x1cc7550, evalfunc_private = 0x0, steps_len = 3, steps_alloc = 16, 
  parent = 0x1c8f1f0, ext_params = 0x0, innermost_caseval = 0x0, innermost_casenull = 0x0, innermost_domainval = 0x0, 
  innermost_domainnull = 0x0}
(gdb) p state->steps[1]
$18 = {opcode = 6, resvalue = 0x1c8fd00, resnull = 0x1c90019, d = {fetch = {last_var = 0, known_desc = 0x0}, var = {
      attnum = 0, vartype = 23}, wholerow = {var = 0x1700000000, first = false, slow = false, tupdesc = 0x0, 
      junkFilter = 0x0}, assign_var = {resultnum = 0, attnum = 23}, assign_tmp = {resultnum = 0}, constval = {
      value = 98784247808, isnull = false}, func = {finfo = 0x1700000000, fcinfo_data = 0x0, fn_addr = 0x0, nargs = 0}, 
    boolexpr = {anynull = 0x1700000000, jumpdone = 0}, qualexpr = {jumpdone = 0}, jump = {jumpdone = 0}, nulltest_row = {
      argdesc = 0x1700000000}, param = {paramid = 0, paramtype = 23}, cparam = {paramfunc = 0x1700000000, paramarg = 0x0, 
      paramid = 0, paramtype = 0}, casetest = {value = 0x1700000000, isnull = 0x0}, make_readonly = {value = 0x1700000000, 
      isnull = 0x0}, iocoerce = {finfo_out = 0x1700000000, fcinfo_data_out = 0x0, finfo_in = 0x0, fcinfo_data_in = 0x0}, 
    sqlvaluefunction = {svf = 0x1700000000}, nextvalueexpr = {seqid = 0, seqtypid = 23}, arrayexpr = {
      elemvalues = 0x1700000000, elemnulls = 0x0, nelems = 0, elemtype = 0, elemlength = 0, elembyval = false, 
      elemalign = 0 '\000', multidims = false}, arraycoerce = {elemexprstate = 0x1700000000, resultelemtype = 0, 
      amstate = 0x0}, row = {tupdesc = 0x1700000000, elemvalues = 0x0, elemnulls = 0x0}, rowcompare_step = {
      finfo = 0x1700000000, fcinfo_data = 0x0, fn_addr = 0x0, jumpnull = 0, jumpdone = 0}, rowcompare_final = {rctype = 0}, 
    minmax = {values = 0x1700000000, nulls = 0x0, nelems = 0, op = IS_GREATEST, finfo = 0x0, fcinfo_data = 0x0}, 
    fieldselect = {fieldnum = 0, resulttype = 23, argdesc = 0x0}, fieldstore = {fstore = 0x1700000000, argdesc = 0x0, 
      values = 0x0, nulls = 0x0, ncolumns = 0}, arrayref_subscript = {state = 0x1700000000, off = 0, isupper = false, 
      jumpdone = 0}, arrayref = {state = 0x1700000000}, domaincheck = {
      constraintname = 0x1700000000 <address out="" of="" bounds="">, checkvalue = 0x0, checknull = 0x0, 
      resulttype = 0}, convert_rowtype = {convert = 0x1700000000, indesc = 0x0, outdesc = 0x0, map = 0x0, 
      initialized = false}, scalararrayop = {element_type = 0, useOr = 23, typlen = 0, typbyval = false, 
      typalign = 0 '\000', finfo = 0x0, fcinfo_data = 0x0, fn_addr = 0x0}, xmlexpr = {xexpr = 0x1700000000, 
      named_argvalue = 0x0, named_argnull = 0x0, argvalue = 0x0, argnull = 0x0}, aggref = {astate = 0x1700000000}, 
    grouping_func = {parent = 0x1700000000, clauses = 0x0}, window_func = {wfstate = 0x1700000000}, subplan = {
      sstate = 0x1700000000}, alternative_subplan = {asstate = 0x1700000000}, agg_deserialize = {aggstate = 0x1700000000, 
      fcinfo_data = 0x0, jumpnull = 0}, agg_strict_input_check = {nulls = 0x1700000000, nargs = 0, jumpnull = 0}, 
    agg_init_trans = {aggstate = 0x1700000000, pertrans = 0x0, aggcontext = 0x0, setno = 0, transno = 0, setoff = 0, 
      jumpnull = 0}, agg_strict_trans_check = {aggstate = 0x1700000000, setno = 0, transno = 0, setoff = 0, jumpnull = 0}, 
---Type <return> to continue, or q <return> to quit---
    agg_trans = {aggstate = 0x1700000000, pertrans = 0x0, aggcontext = 0x0, setno = 0, transno = 0, setoff = 0}}}
(gdb) p state->steps[2]
$19 = {opcode = 18, resvalue = 0x1c8f7e0, resnull = 0x1c8f7dd, d = {fetch = {last_var = 29949056, known_desc = 0x1c8fcd8}, 
    var = {attnum = 29949056, vartype = 0}, wholerow = {var = 0x1c8fc80, first = 216, slow = 252, 
      tupdesc = 0x93d60c <int4pl>, junkFilter = 0x2}, assign_var = {resultnum = 29949056, attnum = 0}, assign_tmp = {
      resultnum = 29949056}, constval = {value = 29949056, isnull = 216}, func = {finfo = 0x1c8fc80, 
      fcinfo_data = 0x1c8fcd8, fn_addr = 0x93d60c <int4pl>, nargs = 2}, boolexpr = {anynull = 0x1c8fc80, 
      jumpdone = 29949144}, qualexpr = {jumpdone = 29949056}, jump = {jumpdone = 29949056}, nulltest_row = {
      argdesc = 0x1c8fc80}, param = {paramid = 29949056, paramtype = 0}, cparam = {paramfunc = 0x1c8fc80, 
      paramarg = 0x1c8fcd8, paramid = 9688588, paramtype = 0}, casetest = {value = 0x1c8fc80, isnull = 0x1c8fcd8}, 
    make_readonly = {value = 0x1c8fc80, isnull = 0x1c8fcd8}, iocoerce = {finfo_out = 0x1c8fc80, 
      fcinfo_data_out = 0x1c8fcd8, finfo_in = 0x93d60c <int4pl>, fcinfo_data_in = 0x2}, sqlvaluefunction = {
      svf = 0x1c8fc80}, nextvalueexpr = {seqid = 29949056, seqtypid = 0}, arrayexpr = {elemvalues = 0x1c8fc80, 
      elemnulls = 0x1c8fcd8, nelems = 9688588, elemtype = 0, elemlength = 2, elembyval = false, elemalign = 0 '\000', 
      multidims = false}, arraycoerce = {elemexprstate = 0x1c8fc80, resultelemtype = 29949144, 
      amstate = 0x93d60c <int4pl>}, row = {tupdesc = 0x1c8fc80, elemvalues = 0x1c8fcd8, elemnulls = 0x93d60c <int4pl>}, 
    rowcompare_step = {finfo = 0x1c8fc80, fcinfo_data = 0x1c8fcd8, fn_addr = 0x93d60c <int4pl>, jumpnull = 2, 
      jumpdone = 0}, rowcompare_final = {rctype = 29949056}, minmax = {values = 0x1c8fc80, nulls = 0x1c8fcd8, 
      nelems = 9688588, op = IS_GREATEST, finfo = 0x2, fcinfo_data = 0x0}, fieldselect = {fieldnum = -896, resulttype = 0, 
      argdesc = 0x1c8fcd8}, fieldstore = {fstore = 0x1c8fc80, argdesc = 0x1c8fcd8, values = 0x93d60c <int4pl>, nulls = 0x2, 
      ncolumns = 0}, arrayref_subscript = {state = 0x1c8fc80, off = 29949144, isupper = false, jumpdone = 9688588}, 
    arrayref = {state = 0x1c8fc80}, domaincheck = {constraintname = 0x1c8fc80 "\f֓", checkvalue = 0x1c8fcd8, 
      checknull = 0x93d60c <int4pl>, resulttype = 2}, convert_rowtype = {convert = 0x1c8fc80, indesc = 0x1c8fcd8, 
      outdesc = 0x93d60c <int4pl>, map = 0x2, initialized = false}, scalararrayop = {element_type = 29949056, 
      useOr = false, typlen = 0, typbyval = 216, typalign = -4 '\374', finfo = 0x93d60c <int4pl>, fcinfo_data = 0x2, 
      fn_addr = 0x0}, xmlexpr = {xexpr = 0x1c8fc80, named_argvalue = 0x1c8fcd8, named_argnull = 0x93d60c <int4pl>, 
      argvalue = 0x2, argnull = 0x0}, aggref = {astate = 0x1c8fc80}, grouping_func = {parent = 0x1c8fc80, 
      clauses = 0x1c8fcd8}, window_func = {wfstate = 0x1c8fc80}, subplan = {sstate = 0x1c8fc80}, alternative_subplan = {
      asstate = 0x1c8fc80}, agg_deserialize = {aggstate = 0x1c8fc80, fcinfo_data = 0x1c8fcd8, jumpnull = 9688588}, 
---Type <return> to continue, or q <return> to quit---
    agg_strict_input_check = {nulls = 0x1c8fc80, nargs = 29949144, jumpnull = 0}, agg_init_trans = {aggstate = 0x1c8fc80, 
      pertrans = 0x1c8fcd8, aggcontext = 0x93d60c <int4pl>, setno = 2, transno = 0, setoff = 0, jumpnull = 0}, 
    agg_strict_trans_check = {aggstate = 0x1c8fc80, setno = 29949144, transno = 0, setoff = 9688588, jumpnull = 0}, 
    agg_trans = {aggstate = 0x1c8fc80, pertrans = 0x1c8fcd8, aggcontext = 0x93d60c <int4pl>, setno = 2, transno = 0, 
      setoff = 0}}}
(gdb) 
```
壓入對應該表示式列的編號,opcode = 14,即EEOP_ASSIGN_TMP
```
(gdb) n
463             scratch.d.assign_tmp.resultnum = tle->resno - 1;
(gdb) 
464             ExprEvalPushStep(state, &scratch);
(gdb) 
(gdb) 
374     foreach(lc, targetList)
(gdb) p *state
$20 = {tag = {type = T_ExprState}, flags = 0 '\000', resnull = false, resvalue = 0, resultslot = 0x1c8f710, 
  steps = 0x1c8f868, evalfunc = 0x0, expr = 0x1cc7550, evalfunc_private = 0x0, steps_len = 4, steps_alloc = 16, 
  parent = 0x1c8f1f0, ext_params = 0x0, innermost_caseval = 0x0, innermost_casenull = 0x0, innermost_domainval = 0x0, 
  innermost_domainnull = 0x0}
(gdb) p state->steps[3]
$21 = {opcode = 14, resvalue = 0x0, resnull = 0x0, d = {fetch = {last_var = 0, known_desc = 0x0}, var = {attnum = 0, 
      vartype = 0}, wholerow = {var = 0x0, first = false, slow = false, tupdesc = 0x0, junkFilter = 0x0}, assign_var = {
      resultnum = 0, attnum = 0}, assign_tmp = {resultnum = 0}, constval = {value = 0, isnull = false}, func = {
      finfo = 0x0, fcinfo_data = 0x0, fn_addr = 0x0, nargs = 0}, boolexpr = {anynull = 0x0, jumpdone = 0}, qualexpr = {
      jumpdone = 0}, jump = {jumpdone = 0}, nulltest_row = {argdesc = 0x0}, param = {paramid = 0, paramtype = 0}, cparam = {
      paramfunc = 0x0, paramarg = 0x0, paramid = 0, paramtype = 0}, casetest = {value = 0x0, isnull = 0x0}, 
    make_readonly = {value = 0x0, isnull = 0x0}, iocoerce = {finfo_out = 0x0, fcinfo_data_out = 0x0, finfo_in = 0x0, 
      fcinfo_data_in = 0x0}, sqlvaluefunction = {svf = 0x0}, nextvalueexpr = {seqid = 0, seqtypid = 0}, arrayexpr = {
      elemvalues = 0x0, elemnulls = 0x0, nelems = 0, elemtype = 0, elemlength = 0, elembyval = false, elemalign = 0 '\000', 
      multidims = false}, arraycoerce = {elemexprstate = 0x0, resultelemtype = 0, amstate = 0x0}, row = {tupdesc = 0x0, 
      elemvalues = 0x0, elemnulls = 0x0}, rowcompare_step = {finfo = 0x0, fcinfo_data = 0x0, fn_addr = 0x0, jumpnull = 0, 
      jumpdone = 0}, rowcompare_final = {rctype = 0}, minmax = {values = 0x0, nulls = 0x0, nelems = 0, op = IS_GREATEST, 
      finfo = 0x0, fcinfo_data = 0x0}, fieldselect = {fieldnum = 0, resulttype = 0, argdesc = 0x0}, fieldstore = {
      fstore = 0x0, argdesc = 0x0, values = 0x0, nulls = 0x0, ncolumns = 0}, arrayref_subscript = {state = 0x0, off = 0, 
      isupper = false, jumpdone = 0}, arrayref = {state = 0x0}, domaincheck = {constraintname = 0x0, checkvalue = 0x0, 
      checknull = 0x0, resulttype = 0}, convert_rowtype = {convert = 0x0, indesc = 0x0, outdesc = 0x0, map = 0x0, 
      initialized = false}, scalararrayop = {element_type = 0, useOr = false, typlen = 0, typbyval = false, 
      typalign = 0 '\000', finfo = 0x0, fcinfo_data = 0x0, fn_addr = 0x0}, xmlexpr = {xexpr = 0x0, named_argvalue = 0x0, 
      named_argnull = 0x0, argvalue = 0x0, argnull = 0x0}, aggref = {astate = 0x0}, grouping_func = {parent = 0x0, 
      clauses = 0x0}, window_func = {wfstate = 0x0}, subplan = {sstate = 0x0}, alternative_subplan = {asstate = 0x0}, 
    agg_deserialize = {aggstate = 0x0, fcinfo_data = 0x0, jumpnull = 0}, agg_strict_input_check = {nulls = 0x0, nargs = 0, 
      jumpnull = 0}, agg_init_trans = {aggstate = 0x0, pertrans = 0x0, aggcontext = 0x0, setno = 0, transno = 0, 
      setoff = 0, jumpnull = 0}, agg_strict_trans_check = {aggstate = 0x0, setno = 0, transno = 0, setoff = 0, 
      jumpnull = 0}, agg_trans = {aggstate = 0x0, pertrans = 0x0, aggcontext = 0x0, setno = 0, transno = 0, setoff = 0}}}
(gdb) 
```
繼續處理下一個列,這是一個"安全"列,壓入EEOP_ASSIGN_SCAN_VAR步驟
```
(gdb) n
376         TargetEntry *tle = lfirst_node(TargetEntry, lc);
(gdb) 
377         Var        *variable = NULL;
(gdb) p *tle
$22 = {xpr = {type = T_TargetEntry}, expr = 0x1c9aa40, resno = 2, resname = 0x1ba7a40 "c2", ressortgroupref = 0, 
  resorigtbl = 237600, resorigcol = 2, resjunk = false}
(gdb) n
378         AttrNumber  attnum = 0;
(gdb) 
379         bool        isSafeVar = false;
(gdb) 
389         if (tle->expr != NULL &&
(gdb) 
390             IsA(tle->expr, Var) &&
(gdb) 
389         if (tle->expr != NULL &&
(gdb) 
391             ((Var *) tle->expr)->varattno > 0)
(gdb) 
390             IsA(tle->expr, Var) &&
(gdb) 
394             variable = (Var *) tle->expr;
(gdb) 
395             attnum = variable->varattno;
(gdb) 
397             if (inputDesc == NULL)
(gdb) 
399             else if (attnum <= inputDesc->natts)
(gdb) 
401                 Form_pg_attribute attr = TupleDescAttr(inputDesc, attnum - 1);
(gdb) 
408                 if (!attr->attisdropped && variable->vartype == attr->atttypid)
(gdb) 
410                     isSafeVar = true;
(gdb) 
415         if (isSafeVar)
(gdb) 
418             switch (variable->varno)
(gdb) 
434                     scratch.opcode = EEOP_ASSIGN_SCAN_VAR;
(gdb) 
435                     break;
(gdb) 
438             scratch.d.assign_var.attnum = attnum - 1;
(gdb) 
439             scratch.d.assign_var.resultnum = tle->resno - 1;
(gdb) 
440             ExprEvalPushStep(state, &scratch);
(gdb) p *state
$23 = {tag = {type = T_ExprState}, flags = 0 '\000', resnull = false, resvalue = 0, resultslot = 0x1c8f710, 
  steps = 0x1c8f868, evalfunc = 0x0, expr = 0x1cc7550, evalfunc_private = 0x0, steps_len = 4, steps_alloc = 16, 
  parent = 0x1c8f1f0, ext_params = 0x0, innermost_caseval = 0x0, innermost_casenull = 0x0, innermost_domainval = 0x0, 
  innermost_domainnull = 0x0}
(gdb) n
374     foreach(lc, targetList)
(gdb) p *state
$24 = {tag = {type = T_ExprState}, flags = 0 '\000', resnull = false, resvalue = 0, resultslot = 0x1c8f710, 
  steps = 0x1c8f868, evalfunc = 0x0, expr = 0x1cc7550, evalfunc_private = 0x0, steps_len = 5, steps_alloc = 16, 
  parent = 0x1c8f1f0, ext_params = 0x0, innermost_caseval = 0x0, innermost_casenull = 0x0, innermost_domainval = 0x0, 
  innermost_domainnull = 0x0}
(gdb) p state->steps[4]
$25 = {opcode = 13, resvalue = 0x0, resnull = 0x0, d = {fetch = {last_var = 1, known_desc = 0x0}, var = {attnum = 1, 
      vartype = 1}, wholerow = {var = 0x100000001, first = false, slow = false, tupdesc = 0x0, junkFilter = 0x0}, 
    assign_var = {resultnum = 1, attnum = 1}, assign_tmp = {resultnum = 1}, constval = {value = 4294967297, 
      isnull = false}, func = {finfo = 0x100000001, fcinfo_data = 0x0, fn_addr = 0x0, nargs = 0}, boolexpr = {
      anynull = 0x100000001, jumpdone = 0}, qualexpr = {jumpdone = 1}, jump = {jumpdone = 1}, nulltest_row = {
      argdesc = 0x100000001}, param = {paramid = 1, paramtype = 1}, cparam = {paramfunc = 0x100000001, paramarg = 0x0, 
      paramid = 0, paramtype = 0}, casetest = {value = 0x100000001, isnull = 0x0}, make_readonly = {value = 0x100000001, 
      isnull = 0x0}, iocoerce = {finfo_out = 0x100000001, fcinfo_data_out = 0x0, finfo_in = 0x0, fcinfo_data_in = 0</int4pl></int4pl></return></return></int4pl></int4pl></int4pl></int4pl></int4pl></int4pl></int4pl></int4pl></int4pl></int4pl></int4pl></return></return></address></address>

PostgreSQL 原始碼解讀（161）- 查詢#81(如何構建表示式解析步驟#1)

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