PostgreSQL 原始碼解讀（56）- 查詢語句#41（make_one_rel函式#6-索...
這一小節主要介紹索引掃描成本估算中get_index_paths函式的主邏輯及其子函式build_index_paths,下一小節介紹子函式build_index_paths中的create_index_path。
一、資料結構
IndexClauseSet
用於收集匹配索引的的條件語句
/* Data structure for collecting qual clauses that match an index */
typedef struct
{
bool nonempty; /* True if lists are not all empty */
/* Lists of RestrictInfos, one per index column */
List *indexclauses[INDEX_MAX_KEYS];
} IndexClauseSet;
二、原始碼解讀
get_index_paths
get_index_paths函式根據給定的索引和索引條件子句,構造索引訪問路徑(IndexPath).
//--------------------------------------------------- get_index_paths
/*
* get_index_paths
* Given an index and a set of index clauses for it, construct IndexPaths.
* 給定索引和索引條件子句,構造索引訪問路徑(IndexPaths).
*
* Plain indexpaths are sent directly to add_path, while potential
* bitmap indexpaths are added to *bitindexpaths for later processing.
* Plain索引訪問路徑直接作為引數傳入到函式add_path中,潛在可能的點陣圖索引路徑
* 被新增到bitindexpaths中供以後處理。
*
* This is a fairly simple frontend to build_index_paths(). Its reason for
* existence is mainly to handle ScalarArrayOpExpr quals properly. If the
* index AM supports them natively, we should just include them in simple
* index paths. If not, we should exclude them while building simple index
* paths, and then make a separate attempt to include them in bitmap paths.
* Furthermore, we should consider excluding lower-order ScalarArrayOpExpr
* quals so as to create ordered paths.
* 該函式簡單構造build_index_paths所需要的引數,並呼叫之.該函式存在的原因是正確
* 處理ScalarArrayOpExpr表示式.
*/
static void
get_index_paths(PlannerInfo *root, RelOptInfo *rel,
IndexOptInfo *index, IndexClauseSet *clauses,
List **bitindexpaths)
{
List *indexpaths;
bool skip_nonnative_saop = false;
bool skip_lower_saop = false;
ListCell *lc;
/*
* Build simple index paths using the clauses. Allow ScalarArrayOpExpr
* clauses only if the index AM supports them natively, and skip any such
* clauses for index columns after the first (so that we produce ordered
* paths if possible).
*/
indexpaths = build_index_paths(root, rel,
index, clauses,
index->predOK,
ST_ANYSCAN,
&skip_nonnative_saop,
&skip_lower_saop);//呼叫build_index_paths函式
/*
* If we skipped any lower-order ScalarArrayOpExprs on an index with an AM
* that supports them, then try again including those clauses. This will
* produce paths with more selectivity but no ordering.
*/
if (skip_lower_saop)
{
indexpaths = list_concat(indexpaths,
build_index_paths(root, rel,
index, clauses,
index->predOK,
ST_ANYSCAN,
&skip_nonnative_saop,
NULL));
}
/*
* Submit all the ones that can form plain IndexScan plans to add_path. (A
* plain IndexPath can represent either a plain IndexScan or an
* IndexOnlyScan, but for our purposes here that distinction does not
* matter. However, some of the indexes might support only bitmap scans,
* and those we mustn't submit to add_path here.)
* 逐一把可以形成Plain索引掃描計劃的的訪問路徑作為引數呼叫add_path
* (plain IndexPath可以是常規的索引掃描或者是IndexOnlyScan)
*
* Also, pick out the ones that are usable as bitmap scans. For that, we
* must discard indexes that don't support bitmap scans, and we also are
* only interested in paths that have some selectivity; we should discard
* anything that was generated solely for ordering purposes.
* 找出可用於點陣圖掃描的索引
*/
foreach(lc, indexpaths)//遍歷訪問路徑
{
IndexPath *ipath = (IndexPath *) lfirst(lc);
if (index->amhasgettuple)
add_path(rel, (Path *) ipath);//呼叫add_path
if (index->amhasgetbitmap &&
(ipath->path.pathkeys == NIL ||
ipath->indexselectivity < 1.0))
*bitindexpaths = lappend(*bitindexpaths, ipath);//如可以,新增到點陣圖索引掃描路徑連結串列中
}
/*
* If there were ScalarArrayOpExpr clauses that the index can't handle
* natively, generate bitmap scan paths relying on executor-managed
* ScalarArrayOpExpr.
*/
if (skip_nonnative_saop)
{
indexpaths = build_index_paths(root, rel,
index, clauses,
false,
ST_BITMAPSCAN,
NULL,
NULL);
*bitindexpaths = list_concat(*bitindexpaths, indexpaths);
}
}
//----------------------------------------------------------- build_index_paths
/*
* build_index_paths
* Given an index and a set of index clauses for it, construct zero
* or more IndexPaths. It also constructs zero or more partial IndexPaths.
* 給定索引和該索引的條件,構造0-N個索引訪問路徑或partial索引訪問路徑(用於並行)
*
* We return a list of paths because (1) this routine checks some cases
* that should cause us to not generate any IndexPath, and (2) in some
* cases we want to consider both a forward and a backward scan, so as
* to obtain both sort orders. Note that the paths are just returned
* to the caller and not immediately fed to add_path().
* 函式返回訪問路徑連結串列:(1)執行過程中的檢查會導致產生不了索引訪問路徑
* (2)在某些情況下，同時考慮正向/反向掃描，以便獲得兩種排序順序。
* 注意:訪問路徑返回給呼叫方,不會馬上反饋到函式add_path中
*
* At top level, useful_predicate should be exactly the index's predOK flag
* (ie, true if it has a predicate that was proven from the restriction
* clauses). When working on an arm of an OR clause, useful_predicate
* should be true if the predicate required the current OR list to be proven.
* Note that this routine should never be called at all if the index has an
* unprovable predicate.
* 在頂層,useful_predicate標記應與索引的predOK標記一致.在操作OR自己的arm(?)時,
* 如果謂詞需要當前的OR連結串列證明,則useful_predicate應為T.
* 注意:如果索引有一個未經驗證的謂詞,則該例程不能被呼叫.
*
* scantype indicates whether we want to create plain indexscans, bitmap
* indexscans, or both. When it's ST_BITMAPSCAN, we will not consider
* index ordering while deciding if a Path is worth generating.
* scantype:提示是否建立plain/bitmap或者兩者兼顧的索引掃描.
* 如該引數值為ST_BITMAPSCAN,則在決定訪問路徑是否產生時不會考慮使用索引排序
*
* If skip_nonnative_saop is non-NULL, we ignore ScalarArrayOpExpr clauses
* unless the index AM supports them directly, and we set *skip_nonnative_saop
* to true if we found any such clauses (caller must initialize the variable
* to false). If it's NULL, we do not ignore ScalarArrayOpExpr clauses.
* skip_nonnative_saop:
* 如為NOT NULL,除非索引的訪問方法(AM)直接支援,否則會忽略
* ScalarArrayOpExpr子句,如支援,則更新skip_nonnative_saop標記為T.
* 如為NULL,不能忽略ScalarArrayOpExpr子句.
*
* If skip_lower_saop is non-NULL, we ignore ScalarArrayOpExpr clauses for
* non-first index columns, and we set *skip_lower_saop to true if we found
* any such clauses (caller must initialize the variable to false). If it's
* NULL, we do not ignore non-first ScalarArrayOpExpr clauses, but they will
* result in considering the scan's output to be unordered.
* skip_lower_saop:
* 如為NOT NULL,ScalarArrayOpExpr子句中的首列不是索引列,則忽略之,
* 同時如果找到相應的子句,則設定skip_lower_saop標記為T.
* 如為NULL,除首個ScalarArrayOpExpr子句外,其他子句不能被忽略,但輸出時不作排序
*
* 輸入/輸出引數:
* 'rel' is the index's heap relation
* rel-相應的RelOptInfo
* 'index' is the index for which we want to generate paths
* index-相應的索引IndexOptInfo
* 'clauses' is the collection of indexable clauses (RestrictInfo nodes)
* clauses-RestrictInfo節點集合
* 'useful_predicate' indicates whether the index has a useful predicate
* useful_predicate-提示索引是否有可用的謂詞
* 'scantype' indicates whether we need plain or bitmap scan support
* scantype-掃描型別,提示是否需要plain/bitmap掃描支援
* 'skip_nonnative_saop' indicates whether to accept SAOP if index AM doesn't
* skip_nonnative_saop-提示是否接受SAOP
* 'skip_lower_saop' indicates whether to accept non-first-column SAOP
* skip_lower_saop-提示是否接受非首列SAOP
*/
static List *
build_index_paths(PlannerInfo *root, RelOptInfo *rel,
IndexOptInfo *index, IndexClauseSet *clauses,
bool useful_predicate,
ScanTypeControl scantype,
bool *skip_nonnative_saop,
bool *skip_lower_saop)
{
List *result = NIL;
IndexPath *ipath;
List *index_clauses;
List *clause_columns;
Relids outer_relids;
double loop_count;
List *orderbyclauses;
List *orderbyclausecols;
List *index_pathkeys;
List *useful_pathkeys;
bool found_lower_saop_clause;
bool pathkeys_possibly_useful;
bool index_is_ordered;
bool index_only_scan;
int indexcol;
/*
* Check that index supports the desired scan type(s)
*/
switch (scantype)
{
case ST_INDEXSCAN:
if (!index->amhasgettuple)
return NIL;
break;
case ST_BITMAPSCAN:
if (!index->amhasgetbitmap)
return NIL;
break;
case ST_ANYSCAN:
/* either or both are OK */
break;
}
/*
* 1. Collect the index clauses into a single list.
* 1. 收集索引子句到單獨的連結串列中
*
* We build a list of RestrictInfo nodes for clauses to be used with this
* index, along with an integer list of the index column numbers (zero
* based) that each clause should be used with. The clauses are ordered
* by index key, so that the column numbers form a nondecreasing sequence.
* (This order is depended on by btree and possibly other places.) The
* lists can be empty, if the index AM allows that.
* 我們為將與此索引一起使用的子句構建了一個RestrictInfo節點連結串列，
* 以及每個子句應該與之一起使用的索引列編號(從0開始)的整數連結串列.
* 子句是按索引鍵排序的，因此列號形成一個非遞減序列。
* (這個排序取決於BTree和可能的其他地方).
* 如果索引訪問方法(AM)允許,連結串列可為空.
*
* found_lower_saop_clause is set true if we accept a ScalarArrayOpExpr
* index clause for a non-first index column. This prevents us from
* assuming that the scan result is ordered. (Actually, the result is
* still ordered if there are equality constraints for all earlier
* columns, but it seems too expensive and non-modular for this code to be
* aware of that refinement.)
*
* We also build a Relids set showing which outer rels are required by the
* selected clauses. Any lateral_relids are included in that, but not
* otherwise accounted for.
* 建立一個已選擇的子句所依賴外部rels的Relids集合,包括lateral_relids.
*/
index_clauses = NIL;
clause_columns = NIL;
found_lower_saop_clause = false;
outer_relids = bms_copy(rel->lateral_relids);
for (indexcol = 0; indexcol < index->ncolumns; indexcol++)//遍歷索引列
{
ListCell *lc;
foreach(lc, clauses->indexclauses[indexcol])//遍歷該列所對應的約束條件
{
RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);//約束條件
if (IsA(rinfo->clause, ScalarArrayOpExpr))//ScalarArrayOpExpr,TODO
{
if (!index->amsearcharray)
{
if (skip_nonnative_saop)
{
/* Ignore because not supported by index */
*skip_nonnative_saop = true;
continue;
}
/* Caller had better intend this only for bitmap scan */
Assert(scantype == ST_BITMAPSCAN);
}
if (indexcol > 0)
{
if (skip_lower_saop)
{
/* Caller doesn't want to lose index ordering */
*skip_lower_saop = true;
continue;
}
found_lower_saop_clause = true;
}
}
index_clauses = lappend(index_clauses, rinfo);//新增到連結串列中
clause_columns = lappend_int(clause_columns, indexcol);
outer_relids = bms_add_members(outer_relids,
rinfo->clause_relids);
}
/*
* If no clauses match the first index column, check for amoptionalkey
* restriction. We can't generate a scan over an index with
* amoptionalkey = false unless there's at least one index clause.
* (When working on columns after the first, this test cannot fail. It
* is always okay for columns after the first to not have any
* clauses.)
*/
if (index_clauses == NIL && !index->amoptionalkey)//沒有約束條件,返回空指標
return NIL;
}
/* We do not want the index's rel itself listed in outer_relids */
outer_relids = bms_del_member(outer_relids, rel->relid);//去掉自身relid
/* Enforce convention that outer_relids is exactly NULL if empty */
if (bms_is_empty(outer_relids))
outer_relids = NULL;//設定為NULL
/* Compute loop_count for cost estimation purposes */
//計算成本估算所需要的迴圈次數loop_count
loop_count = get_loop_count(root, rel->relid, outer_relids);
/*
* 2. Compute pathkeys describing index's ordering, if any, then see how
* many of them are actually useful for this query. This is not relevant
* if we are only trying to build bitmap indexscans, nor if we have to
* assume the scan is unordered.
* 2.計算描述索引排序的路徑鍵(如果有的話)，如存在的話,檢查有多少對查詢有用。
* 如果只是嘗試構建點陣圖索引掃描，或者掃描是無序的，那就無關緊要了。
*/
pathkeys_possibly_useful = (scantype != ST_BITMAPSCAN &&
!found_lower_saop_clause &&
has_useful_pathkeys(root, rel));//是否存在可用的Pathkeys
index_is_ordered = (index->sortopfamily != NULL);//索引是否排序?
if (index_is_ordered && pathkeys_possibly_useful)//索引排序&存在可用的Pathkeys
{
index_pathkeys = build_index_pathkeys(root, index,
ForwardScanDirection);//建立正向(ForwardScanDirection)掃描排序鍵
useful_pathkeys = truncate_useless_pathkeys(root, rel,
index_pathkeys);//截斷無用的Pathkeys
orderbyclauses = NIL;
orderbyclausecols = NIL;
}
else if (index->amcanorderbyop && pathkeys_possibly_useful)//訪問方法可以透過運算子排序
{
/* see if we can generate ordering operators for query_pathkeys */
match_pathkeys_to_index(index, root->query_pathkeys,
&orderbyclauses,
&orderbyclausecols);//是否可以生成排序運算子
if (orderbyclauses)
useful_pathkeys = root->query_pathkeys;//如可以,則賦值
else
useful_pathkeys = NIL;
}
else//設定為NULL
{
useful_pathkeys = NIL;
orderbyclauses = NIL;
orderbyclausecols = NIL;
}
/*
* 3. Check if an index-only scan is possible. If we're not building
* plain indexscans, this isn't relevant since bitmap scans don't support
* index data retrieval anyway.
* 3. 檢查是否只需要掃描索引.如果我們不構建純索引掃描，這是無關緊要的,
* 因為點陣圖掃描不支援索引資料檢索。
*/
index_only_scan = (scantype != ST_BITMAPSCAN &&
check_index_only(rel, index));
/*
* 4. Generate an indexscan path if there are relevant restriction clauses
* in the current clauses, OR the index ordering is potentially useful for
* later merging or final output ordering, OR the index has a useful
* predicate, OR an index-only scan is possible.
* 4. 如果當前子句中有相關的限制子句，或者索引排序對於以後的
* 合併或最終的輸出排序可能有用，或者索引存在可用的謂詞，
* 或者可能進行純索引掃描，則生成索引掃描路徑。
*/
if (index_clauses != NIL || useful_pathkeys != NIL || useful_predicate ||
index_only_scan)
{
ipath = create_index_path(root, index,
index_clauses,
clause_columns,
orderbyclauses,
orderbyclausecols,
useful_pathkeys,
index_is_ordered ?
ForwardScanDirection :
NoMovementScanDirection,
index_only_scan,
outer_relids,
loop_count,
false);//建立索引掃描路徑
result = lappend(result, ipath);//新增到結果連結串列中
/*
* If appropriate, consider parallel index scan. We don't allow
* parallel index scan for bitmap index scans.
* 如果合適，考慮並行索引掃描。如為點陣圖索引,則不能使用並行.。
*/
if (index->amcanparallel &&
rel->consider_parallel && outer_relids == NULL &&
scantype != ST_BITMAPSCAN)
{
ipath = create_index_path(root, index,
index_clauses,
clause_columns,
orderbyclauses,
orderbyclausecols,
useful_pathkeys,
index_is_ordered ?
ForwardScanDirection :
NoMovementScanDirection,
index_only_scan,
outer_relids,
loop_count,
true);//建立並行索引掃描路徑
/*
* if, after costing the path, we find that it's not worth using
* parallel workers, just free it.
*/
if (ipath->path.parallel_workers > 0)//在worker>0的情況下
add_partial_path(rel, (Path *) ipath);//新增partial路徑
else
pfree(ipath);
}
}
/*
* 5. If the index is ordered, a backwards scan might be interesting.
* 5. 如果索引是已排序的(如BTree等),構建反向掃描(BackwardScanDirection)路徑
*/
if (index_is_ordered && pathkeys_possibly_useful)
{
index_pathkeys = build_index_pathkeys(root, index,
BackwardScanDirection);
useful_pathkeys = truncate_useless_pathkeys(root, rel,
index_pathkeys);
if (useful_pathkeys != NIL)
{
ipath = create_index_path(root, index,
index_clauses,
clause_columns,
NIL,
NIL,
useful_pathkeys,
BackwardScanDirection,
index_only_scan,
outer_relids,
loop_count,
false);
result = lappend(result, ipath);
/* If appropriate, consider parallel index scan */
if (index->amcanparallel &&
rel->consider_parallel && outer_relids == NULL &&
scantype != ST_BITMAPSCAN)
{
ipath = create_index_path(root, index,
index_clauses,
clause_columns,
NIL,
NIL,
useful_pathkeys,
BackwardScanDirection,
index_only_scan,
outer_relids,
loop_count,
true);
/*
* if, after costing the path, we find that it's not worth
* using parallel workers, just free it.
*/
if (ipath->path.parallel_workers > 0)
add_partial_path(rel, (Path *) ipath);
else
pfree(ipath);
}
}
}
return result;
}
三、跟蹤分析
測試指令碼如下
select a.*,b.grbh,b.je
from t_dwxx a,
lateral (select t1.dwbh,t1.grbh,t2.je
from t_grxx t1
inner join t_jfxx t2 on t1.dwbh = a.dwbh and t1.grbh = t2.grbh) b
where a.dwbh = '1001'
order by b.dwbh;
啟動gdb
(gdb) b get_index_paths
Breakpoint 1 at 0x74db5e: file indxpath.c, line 740.
(gdb) c
Continuing.
Breakpoint 1, get_index_paths (root=0x2704720, rel=0x27041b0, index=0x2713898, clauses=0x7fff834d8090,
bitindexpaths=0x7fff834d81b0) at indxpath.c:740
740 bool skip_nonnative_saop = false;
選擇t_dwxx的主鍵進行跟蹤
(gdb) p *index
$2 = {type = T_IndexOptInfo, indexoid = 16738, reltablespace = 0, rel = 0x27041b0, pages = 30, tuples = 10000,
tree_height = 1, ncolumns = 1, nkeycolumns = 1, indexkeys = 0x2713bd8, indexcollations = 0x2713bf0, opfamily = 0x2713c08,
opcintype = 0x2713c20, sortopfamily = 0x2713c08, reverse_sort = 0x2713c50, nulls_first = 0x2713c68,
canreturn = 0x2713c38, relam = 403, indexprs = 0x0, indpred = 0x0, indextlist = 0x2713ba8, indrestrictinfo = 0x2716c18,
predOK = false, unique = true, immediate = true, hypothetical = false, amcanorderbyop = false, amoptionalkey = true,
amsearcharray = true, amsearchnulls = true, amhasgettuple = true, amhasgetbitmap = true, amcanparallel = true,
amcostestimate = 0x94f0ad <btcostestimate>}
--
testdb=# select relname from pg_class where oid=16738;
relname
-------------
t_dwxx_pkey
(1 row)
--
進入函式build_index_paths
(gdb) step
build_index_paths (root=0x2704720, rel=0x27041b0, index=0x27135b8, clauses=0x7fff834d8090, useful_predicate=false,
scantype=ST_ANYSCAN, skip_nonnative_saop=0x7fff834d7e27, skip_lower_saop=0x7fff834d7e26) at indxpath.c:864
864 List *result = NIL;
檢視輸入引數,其中clauses中的indexclauses陣列,儲存約束條件連結串列
(gdb) p *clauses
$3 = {nonempty = true, indexclauses = {0x2717728, 0x0 <repeats 31 times>}}
(gdb) set $ri=(RestrictInfo *)clauses->indexclauses[0]->head->data.ptr_value
連結串列的第一個約束條件為:t_dwxx.dwbh(varno = 1, varattno = 2)='1001'(constvalue = 40986784)
(gdb) p *((OpExpr *)$ri->clause)->args
$11 = {type = T_List, length = 2, head = 0x27169f8, tail = 0x27169a8}
(gdb) p *(Node *)((OpExpr *)$ri->clause)->args->head->data.ptr_value
$12 = {type = T_RelabelType}
(gdb) p *(Node *)((OpExpr *)$ri->clause)->args->head->next->data.ptr_value
$13 = {type = T_Const}
(gdb) set $tmp1=(RelabelType *)((OpExpr *)$ri->clause)->args->head->data.ptr_value
(gdb) set $tmp2=(Const *)((OpExpr *)$ri->clause)->args->head->next->data.ptr_value
(gdb) p *(Var *)$tmp1->arg
$17 = {xpr = {type = T_Var}, varno = 1, varattno = 2, vartype = 1043, vartypmod = 24, varcollid = 100, varlevelsup = 0,
varnoold = 1, varoattno = 2, location = 147}
(gdb) p *(Const *)$tmp2
$18 = {xpr = {type = T_Const}, consttype = 25, consttypmod = -1, constcollid = 100, constlen = -1, constvalue = 40986784,
constisnull = false, constbyval = false, location = 194}
掃描型別,ST_ANYSCAN,包括plain&bitmap
(gdb) n
883 switch (scantype)
(gdb) p scantype
$19 = ST_ANYSCAN
Step 1:收集索引子句到單獨的連結串列中
923 for (indexcol = 0; indexcol < index->ncolumns; indexcol++)
(gdb) p outer_relids
$20 = (Relids) 0x0
(gdb) n
927 foreach(lc, clauses->indexclauses[indexcol])
(gdb) p indexcol
$21 = 0
(gdb) n
#rinfo約束條件:t_dwxx.dwbh='1001'
929 RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
(gdb)
931 if (IsA(rinfo->clause, ScalarArrayOpExpr))
(gdb)
Step 1的主要邏輯:
(gdb) n
955 index_clauses = lappend(index_clauses, rinfo);
(gdb) p *index_clauses
Cannot access memory at address 0x0
(gdb) n
956 clause_columns = lappend_int(clause_columns, indexcol);
(gdb)
958 rinfo->clause_relids);
(gdb)
957 outer_relids = bms_add_members(outer_relids,
Step 1完成後:
(gdb) p *outer_relids
$23 = {nwords = 1, words = 0x27177fc}
(gdb) p *index_clauses
$26 = {type = T_List, length = 1, head = 0x2717758, tail = 0x2717758}
(gdb) p outer_relids->words[0]
$27 = 0 -->無外部的Relids
(gdb) p *clause_columns
$31 = {type = T_IntList, length = 1, head = 0x27177a8, tail = 0x27177a8}
(gdb) p clause_columns->head->data.int_value
$32 = 0 -->列陣列編號為0
迴圈次數:
(gdb) p loop_count
$33 = 1
Step 2:計算描述索引排序的路徑鍵(如果有的話)，如存在的話,檢查有多少對查詢有用。
...
(gdb) p pathkeys_possibly_useful
$35 = true
(gdb) p index_is_ordered
$36 = true
建立正向掃描排序鍵
(gdb) n
994 index_pathkeys = build_index_pathkeys(root, index,
(gdb) p *index_pathkeys
Cannot access memory at address 0x0 -->無需排序
Step 3:檢查是否只需要掃描索引
(gdb) p index_only_scan
$37 = false -->No way!
Step 4:生成索引掃描路徑
呼叫函式create_index_path(下節介紹)
1036 ipath = create_index_path(root, index,
(gdb)
1049 result = lappend(result, ipath);
(gdb) p *ipath
$38 = {path = {type = T_IndexPath, pathtype = T_IndexScan, parent = 0x27041b0, pathtarget = 0x27134c8, param_info = 0x0,
parallel_aware = false, parallel_safe = true, parallel_workers = 0, rows = 1, startup_cost = 0.28500000000000003,
total_cost = 8.3025000000000002, pathkeys = 0x0}, indexinfo = 0x27135b8, indexclauses = 0x2717778,
indexquals = 0x27178e8, indexqualcols = 0x2717938, indexorderbys = 0x0, indexorderbycols = 0x0,
indexscandir = ForwardScanDirection, indextotalcost = 4.2925000000000004, indexselectivity = 0.0001}
Step 5:構建反向掃描(BackwardScanDirection)路徑
(gdb) p index_is_ordered
$41 = true
(gdb) p pathkeys_possibly_useful
$42 = true
...
(gdb) p *index_pathkeys
Cannot access memory at address 0x0 -->無需排序
返回結果
1137 return result;
(gdb)
1138 }
至此函式呼叫結束.
四、參考資料
allpaths.c
cost.h
costsize.c
PG Document:Query Planning
來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/6906/viewspace-2374852/，如需轉載，請註明出處，否則將追究法律責任。
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