PostgreSQL 原始碼解讀（66）- 查詢語句#51（make_one_rel函式#16-...
本節大體介紹了動態規劃演算法實現(standard_join_search)中的join_search_one_level->make_join_rel->populate_joinrel_with_paths->add_paths_to_joinrel函式中的sort_inner_and_outer函式，該函式嘗試構造merge join訪問路徑。
merge join的演算法實現虛擬碼如下:
READ data_set_1 SORT BY JOIN KEY TO temp_ds1
READ data_set_2 SORT BY JOIN KEY TO temp_ds2
READ ds1_row FROM temp_ds1
READ ds2_row FROM temp_ds2
WHILE NOT eof ON temp_ds1,temp_ds2 LOOP
IF ( temp_ds1.key = temp_ds2.key ) OUTPUT JOIN ds1_row,ds2_row
ELSIF ( temp_ds1.key <= temp_ds2.key ) READ ds1_row FROM temp_ds1
ELSIF ( temp_ds1.key => temp_ds2.key ) READ ds2_row FROM temp_ds2
END LOOP
一、資料結構
Cost相關
注意:實際使用的引數值透過系統配置檔案定義,而不是這裡的常量定義!
typedef double Cost; /* execution cost (in page-access units) */
/* defaults for costsize.c's Cost parameters */
/* NB: cost-estimation code should use the variables, not these constants! */
/* 注意:實際值透過系統配置檔案定義,而不是這裡的常量定義! */
/* If you change these, update backend/utils/misc/postgresql.sample.conf */
#define DEFAULT_SEQ_PAGE_COST 1.0 //順序掃描page的成本
#define DEFAULT_RANDOM_PAGE_COST 4.0 //隨機掃描page的成本
#define DEFAULT_CPU_TUPLE_COST 0.01 //處理一個元組的CPU成本
#define DEFAULT_CPU_INDEX_TUPLE_COST 0.005 //處理一個索引元組的CPU成本
#define DEFAULT_CPU_OPERATOR_COST 0.0025 //執行一次操作或函式的CPU成本
#define DEFAULT_PARALLEL_TUPLE_COST 0.1 //並行執行,從一個worker傳輸一個元組到另一個worker的成本
#define DEFAULT_PARALLEL_SETUP_COST 1000.0 //構建並行執行環境的成本
#define DEFAULT_EFFECTIVE_CACHE_SIZE 524288 /*先前已有介紹, measured in pages */
double seq_page_cost = DEFAULT_SEQ_PAGE_COST;
double random_page_cost = DEFAULT_RANDOM_PAGE_COST;
double cpu_tuple_cost = DEFAULT_CPU_TUPLE_COST;
double cpu_index_tuple_cost = DEFAULT_CPU_INDEX_TUPLE_COST;
double cpu_operator_cost = DEFAULT_CPU_OPERATOR_COST;
double parallel_tuple_cost = DEFAULT_PARALLEL_TUPLE_COST;
double parallel_setup_cost = DEFAULT_PARALLEL_SETUP_COST;
int effective_cache_size = DEFAULT_EFFECTIVE_CACHE_SIZE;
Cost disable_cost = 1.0e10;//1後面10個0,透過設定一個巨大的成本,讓最佳化器自動放棄此路徑
int max_parallel_workers_per_gather = 2;//每次gather使用的worker數
二、原始碼解讀
sort_inner_and_outer函式嘗試構造merge join訪問路徑.
構造過程中的成本估算實現函式initial_cost_mergejoin和final_cost_mergejoin在下一節介紹.
//------------------------------------------------ sort_inner_and_outer
/*
* sort_inner_and_outer
* Create mergejoin join paths by explicitly sorting both the outer and
* inner join relations on each available merge ordering.
* 顯式排序outer和inner表,建立mergejoin連線路徑.
*
* 'joinrel' is the join relation
* joinrel-連線後新生成的relation
* 'outerrel' is the outer join relation
* outerrel-參與連線的outer relation(俗稱外表,驅動表)
* 'innerrel' is the inner join relation
* innerrel-參與連線的inner relation(俗稱內表)
* 'jointype' is the type of join to do
* jointype-連線型別
* 'extra' contains additional input values
* extra-額外的輸入引數
*/
static void
sort_inner_and_outer(PlannerInfo *root,
RelOptInfo *joinrel,
RelOptInfo *outerrel,
RelOptInfo *innerrel,
JoinType jointype,
JoinPathExtraData *extra)
{
JoinType save_jointype = jointype;
Path *outer_path;
Path *inner_path;
Path *cheapest_partial_outer = NULL;
Path *cheapest_safe_inner = NULL;
List *all_pathkeys;
ListCell *l;
/*
* We only consider the cheapest-total-cost input paths, since we are
* assuming here that a sort is required. We will consider
* cheapest-startup-cost input paths later, and only if they don't need a
* sort.
* 由於我們假定排序是必須的,只考慮總成本最低的路徑。
* 後續會嘗試啟動成本最低的路徑，並且只在它們不需要排序的情況下。
*
* This function intentionally does not consider parameterized input
* paths, except when the cheapest-total is parameterized. If we did so,
* we'd have a combinatorial explosion of mergejoin paths of dubious
* value. This interacts with decisions elsewhere that also discriminate
* against mergejoins with parameterized inputs; see comments in
* src/backend/optimizer/README.
* 該函式特意不考慮引數化輸入路徑，除非成本最低路徑是引數化的。
* 如果考慮了引數化輸入路徑,mergejoin將有一個數量巨大的組合,成本上划不來。
* 而且這將與其他地方的決策相互作用，這些決策同樣會"歧視"使用引數化輸入的mergejoin;
* 請參閱src/backend/optimizer/README中的註釋。
*/
outer_path = outerrel->cheapest_total_path;
inner_path = innerrel->cheapest_total_path;
/*
* If either cheapest-total path is parameterized by the other rel, we
* can't use a mergejoin. (There's no use looking for alternative input
* paths, since these should already be the least-parameterized available
* paths.)
* 如果任意一個總成本最低的路徑使用了引數化訪問路徑，那麼不能使用合併連線。
* (沒有必要尋找替代的輸入路徑，因為這些路徑已經是引數化最少的可用路徑了。)
*/
if (PATH_PARAM_BY_REL(outer_path, innerrel) ||
PATH_PARAM_BY_REL(inner_path, outerrel))
return;
/*
* If unique-ification is requested, do it and then handle as a plain
* inner join.
* 如果需要保證唯一,建立唯一性訪問路徑並設定為JOIN_INNER連線型別
*/
if (jointype == JOIN_UNIQUE_OUTER)
{
outer_path = (Path *) create_unique_path(root, outerrel,
outer_path, extra->sjinfo);
Assert(outer_path);
jointype = JOIN_INNER;
}
else if (jointype == JOIN_UNIQUE_INNER)
{
inner_path = (Path *) create_unique_path(root, innerrel,
inner_path, extra->sjinfo);
Assert(inner_path);
jointype = JOIN_INNER;
}
/*
* If the joinrel is parallel-safe, we may be able to consider a partial
* merge join. However, we can't handle JOIN_UNIQUE_OUTER, because the
* outer path will be partial, and therefore we won't be able to properly
* guarantee uniqueness. Similarly, we can't handle JOIN_FULL and
* JOIN_RIGHT, because they can produce false null extended rows. Also,
* the resulting path must not be parameterized.
* 如果連線可以並行執行，那麼可以考慮並行合併連線。
* 但是，PG不能處理JOIN_UNIQUE_OUTER，因為外部路徑是部分的，因此不能正確地保證惟一性。
* 類似地，PG不能處理JOIN_FULL和JOIN_RIGHT，因為它們會產生假空擴充套件行。
* 此外，生成的路徑不能被引數化。
*/
if (joinrel->consider_parallel &&
save_jointype != JOIN_UNIQUE_OUTER &&
save_jointype != JOIN_FULL &&
save_jointype != JOIN_RIGHT &&
outerrel->partial_pathlist != NIL &&
bms_is_empty(joinrel->lateral_relids))
{
cheapest_partial_outer = (Path *) linitial(outerrel->partial_pathlist);
if (inner_path->parallel_safe)
cheapest_safe_inner = inner_path;
else if (save_jointype != JOIN_UNIQUE_INNER)
cheapest_safe_inner =
get_cheapest_parallel_safe_total_inner(innerrel->pathlist);
}
/*
* Each possible ordering of the available mergejoin clauses will generate
* a differently-sorted result path at essentially the same cost. We have
* no basis for choosing one over another at this level of joining, but
* some sort orders may be more useful than others for higher-level
* mergejoins, so it's worth considering multiple orderings.
* 可用的mergejoin子句的每一種可能的排序都將以基本相同的代價生成不同的排序結果路徑。
* 在這種連線級別上，我們沒有選擇一個排序的基礎，但是對於更高層的合併，
* 某些排序順序可能比其他更有用，因此值得考慮多種排序的順序。
*
* Actually, it's not quite true that every mergeclause ordering will
* generate a different path order, because some of the clauses may be
* partially redundant (refer to the same EquivalenceClasses). Therefore,
* what we do is convert the mergeclause list to a list of canonical
* pathkeys, and then consider different orderings of the pathkeys.
* 實際上，並不是每個mergeclause都會生成不同的路徑順序，因為有些子句可能是部分冗餘的(請參閱等價類)。
* 因此，要做的是將mergeclause列表轉換為一個規範路徑鍵列表，然後考慮路徑鍵的不同順序。
*
* Generating a path for *every* permutation of the pathkeys doesn't seem
* like a winning strategy; the cost in planning time is too high. For
* now, we generate one path for each pathkey, listing that pathkey first
* and the rest in random order. This should allow at least a one-clause
* mergejoin without re-sorting against any other possible mergejoin
* partner path. But if we've not guessed the right ordering of secondary
* keys, we may end up evaluating clauses as qpquals when they could have
* been done as mergeclauses. (In practice, it's rare that there's more
* than two or three mergeclauses, so expending a huge amount of thought
* on that is probably not worth it.)
* 為每個路徑鍵的排列生成路徑似乎不是一個好的策略,因為計劃時間成本太高了。
* 現在，我們為每個pathkey生成一個路徑，首先列出這個pathkey，然後按隨機順序列出其餘的路徑。
* 這應該至少允許一個單子句的mergejoin，而不需要對任何其他可能的mergejoin路徑進行重新排序。
* 但如果沒有正確判斷二級鍵的順序，那麼可能會以qpquals的形式計算條件子句，而這些子句本可以作為mergeclauses完成。
* (實際上，很少有兩個或三個以上的mergeclauses，所以花大量時間在這上面可能不值得)。
*
* The pathkey order returned by select_outer_pathkeys_for_merge() has
* some heuristics behind it (see that function), so be sure to try it
* exactly as-is as well as making variants.
* 透過呼叫select_outer_pathkeys_for_merge函式返回的排序鍵順序pathkey
* 背後有一些啟發式函式(請參閱該函式)，所以一定要按原樣嘗試它，並建立變體。
*/
all_pathkeys = select_outer_pathkeys_for_merge(root,
extra->mergeclause_list,
joinrel);
foreach(l, all_pathkeys)//遍歷所有可用的排序鍵
{
List *front_pathkey = (List *) lfirst(l);
List *cur_mergeclauses;
List *outerkeys;
List *innerkeys;
List *merge_pathkeys;
/* Make a pathkey list with this guy first */
if (l != list_head(all_pathkeys))
outerkeys = lcons(front_pathkey,
list_delete_ptr(list_copy(all_pathkeys),
front_pathkey));
else
outerkeys = all_pathkeys; /* no work at first one... */
/* Sort the mergeclauses into the corresponding ordering */
cur_mergeclauses =
find_mergeclauses_for_outer_pathkeys(root,
outerkeys,
extra->mergeclause_list);
/* Should have used them all... */
Assert(list_length(cur_mergeclauses) == list_length(extra->mergeclause_list));
/* Build sort pathkeys for the inner side */
innerkeys = make_inner_pathkeys_for_merge(root,
cur_mergeclauses,
outerkeys);
/* Build pathkeys representing output sort order */
merge_pathkeys = build_join_pathkeys(root, joinrel, jointype,
outerkeys);
/*
* And now we can make the path.
*
* Note: it's possible that the cheapest paths will already be sorted
* properly. try_mergejoin_path will detect that case and suppress an
* explicit sort step, so we needn't do so here.
* 注意:成本最低的路徑可能已被正確排序。
* try_mergejoin_path將檢測這種情況並禁止顯式排序步驟，因此在這裡不需要這樣做。
*/
try_mergejoin_path(root,
joinrel,
outer_path,
inner_path,
merge_pathkeys,
cur_mergeclauses,
outerkeys,
innerkeys,
jointype,
extra,
false);
/*
* If we have partial outer and parallel safe inner path then try
* partial mergejoin path.
* 並行處理
*/
if (cheapest_partial_outer && cheapest_safe_inner)
try_partial_mergejoin_path(root,
joinrel,
cheapest_partial_outer,
cheapest_safe_inner,
merge_pathkeys,
cur_mergeclauses,
outerkeys,
innerkeys,
jointype,
extra);
}
}
//----------------------------------- try_mergejoin_path
/*
* try_mergejoin_path
* Consider a merge join path; if it appears useful, push it into
* the joinrel's pathlist via add_path().
* 嘗試merge join path,如可用,則把該路徑透過add_path函式放在joinrel的pathlist連結串列中
*/
static void
try_mergejoin_path(PlannerInfo *root,
RelOptInfo *joinrel,
Path *outer_path,
Path *inner_path,
List *pathkeys,
List *mergeclauses,
List *outersortkeys,
List *innersortkeys,
JoinType jointype,
JoinPathExtraData *extra,
bool is_partial)
{
Relids required_outer;
JoinCostWorkspace workspace;
if (is_partial)
{
try_partial_mergejoin_path(root,
joinrel,
outer_path,
inner_path,
pathkeys,
mergeclauses,
outersortkeys,
innersortkeys,
jointype,
extra);//並行執行
return;
}
/*
* Check to see if proposed path is still parameterized, and reject if the
* parameterization wouldn't be sensible.
* 檢查建議的路徑是否仍然引數化，如果引數化不合理，則捨棄。
*/
required_outer = calc_non_nestloop_required_outer(outer_path,
inner_path);
if (required_outer &&
!bms_overlap(required_outer, extra->param_source_rels))
{
/* Waste no memory when we reject a path here */
bms_free(required_outer);
return;
}
/*
* If the given paths are already well enough ordered, we can skip doing
* an explicit sort.
* 如果給定的路徑已完成排序,則跳過顯式排序.
*/
if (outersortkeys &&
pathkeys_contained_in(outersortkeys, outer_path->pathkeys))
outersortkeys = NIL;
if (innersortkeys &&
pathkeys_contained_in(innersortkeys, inner_path->pathkeys))
innersortkeys = NIL;
/*
* See comments in try_nestloop_path().
*/
initial_cost_mergejoin(root, &workspace, jointype, mergeclauses,
outer_path, inner_path,
outersortkeys, innersortkeys,
extra);//初始化mergejoin
if (add_path_precheck(joinrel,
workspace.startup_cost, workspace.total_cost,
pathkeys, required_outer))//執行前置檢查
{
add_path(joinrel, (Path *)
create_mergejoin_path(root,
joinrel,
jointype,
&workspace,
extra,
outer_path,
inner_path,
extra->restrictlist,
pathkeys,
required_outer,
mergeclauses,
outersortkeys,
innersortkeys));//建立並新增路徑
}
else
{
/* Waste no memory when we reject a path here */
bms_free(required_outer);
}
}
//----------------------- create_mergejoin_path
/*
* create_mergejoin_path
* Creates a pathnode corresponding to a mergejoin join between
* two relations
* 建立merge join訪問路徑
*
* 'joinrel' is the join relation
* joinrel-連線後新生成的relation
* 'jointype' is the type of join required
* jointype-連線型別
* 'workspace' is the result from initial_cost_mergejoin
* workspace-透過函式initial_cost_mergejoin返回的結果
* 'extra' contains various information about the join
* extra-額外的輸入引數
* 'outer_path' is the outer path
* outer_path-outer relation訪問路徑
* 'inner_path' is the inner path
* inner_path-inner relation訪問路徑
* 'restrict_clauses' are the RestrictInfo nodes to apply at the join
* restrict_clauses-約束條件子句
* 'pathkeys' are the path keys of the new join path
* pathkeys-排序鍵
* 'required_outer' is the set of required outer rels
* required_outer-如需要outer rels,則在此儲存relids
* 'mergeclauses' are the RestrictInfo nodes to use as merge clauses
* (this should be a subset of the restrict_clauses list)
* mergeclauses-合併連線條件子句
* 'outersortkeys' are the sort varkeys for the outer relation
* outersortkeys-outer relation的排序鍵
* 'innersortkeys' are the sort varkeys for the inner relation
* innersortkeys-inner relation的排序鍵
*/
MergePath *
create_mergejoin_path(PlannerInfo *root,
RelOptInfo *joinrel,
JoinType jointype,
JoinCostWorkspace *workspace,
JoinPathExtraData *extra,
Path *outer_path,
Path *inner_path,
List *restrict_clauses,
List *pathkeys,
Relids required_outer,
List *mergeclauses,
List *outersortkeys,
List *innersortkeys)
{
MergePath *pathnode = makeNode(MergePath);
pathnode->jpath.path.pathtype = T_MergeJoin;
pathnode->jpath.path.parent = joinrel;
pathnode->jpath.path.pathtarget = joinrel->reltarget;
pathnode->jpath.path.param_info =
get_joinrel_parampathinfo(root,
joinrel,
outer_path,
inner_path,
extra->sjinfo,
required_outer,
&restrict_clauses);
pathnode->jpath.path.parallel_aware = false;
pathnode->jpath.path.parallel_safe = joinrel->consider_parallel &&
outer_path->parallel_safe && inner_path->parallel_safe;
/* This is a foolish way to estimate parallel_workers, but for now... */
pathnode->jpath.path.parallel_workers = outer_path->parallel_workers;
pathnode->jpath.path.pathkeys = pathkeys;
pathnode->jpath.jointype = jointype;
pathnode->jpath.inner_unique = extra->inner_unique;
pathnode->jpath.outerjoinpath = outer_path;
pathnode->jpath.innerjoinpath = inner_path;
pathnode->jpath.joinrestrictinfo = restrict_clauses;
pathnode->path_mergeclauses = mergeclauses;
pathnode->outersortkeys = outersortkeys;
pathnode->innersortkeys = innersortkeys;
/* pathnode->skip_mark_restore will be set by final_cost_mergejoin */
/* pathnode->materialize_inner will be set by final_cost_mergejoin */
final_cost_mergejoin(root, pathnode, workspace, extra);//估算成本
return pathnode;
}
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
order by b.dwbh;
啟動gdb,設定斷點
(gdb) b sort_inner_and_outer
Breakpoint 1 at 0x7af63a: file joinpath.c, line 888.
(gdb) c
Continuing.
Breakpoint 1, sort_inner_and_outer (root=0x1a4a278, joinrel=0x1aa7180, outerrel=0x1a55700, innerrel=0x1a56c30,
jointype=JOIN_INNER, extra=0x7ffca933f880) at joinpath.c:888
888 JoinType save_jointype = jointype;
(gdb)
新生成的joinrel是1號和3號RTE的連線,型別為JOIN_INNER
(gdb) p *joinrel->relids->words
$1 = 10
(gdb) p jointype
$2 = JOIN_INNER
獲取排序鍵,PathKey中的等價類EC,成員為t_grxx.dwbh和t_dwxx.dwbh
...
(gdb)
993 all_pathkeys = select_outer_pathkeys_for_merge(root,
(gdb) n
997 foreach(l, all_pathkeys)
(gdb) p *all_pathkeys
$3 = {type = T_List, length = 1, head = 0x1a69490, tail = 0x1a69490}
(gdb) p *(PathKey *)all_pathkeys->head->data.ptr_value
$5 = {type = T_PathKey, pk_eclass = 0x1a60e08, pk_opfamily = 1994, pk_strategy = 1, pk_nulls_first = false}
...
(gdb) set $rt=(RelabelType *)((EquivalenceMember *)$ec->ec_members->head->data.ptr_value)->em_expr
(gdb) p *$rt->arg
$14 = {type = T_Var}
(gdb) p *(Var *)$rt->arg
$15 = {xpr = {type = T_Var}, varno = 3, varattno = 1, vartype = 1043, vartypmod = 14, varcollid = 100, varlevelsup = 0,
varnoold = 3, varoattno = 1, location = 208}
(gdb) set $rt2=(RelabelType *)((EquivalenceMember *)$ec->ec_members->head->next->data.ptr_value)->em_expr
(gdb) p *(Var *)$rt2->arg
$16 = {xpr = {type = T_Var}, varno = 1, varattno = 2, vartype = 1043, vartypmod = 24, varcollid = 100, varlevelsup = 0,
varnoold = 1, varoattno = 2, location = 218}
開始遍歷all_pathkeys
(gdb) n
999 List *front_pathkey = (List *) lfirst(l);
獲取連線條件子句,t_dwxx.dwbh=t_grxx.dwbh
(gdb) p *cur_mergeclauses
$17 = {type = T_List, length = 1, head = 0x1a694f0, tail = 0x1a694f0}
構建outer和inner relation的排序鍵
(gdb) p *(PathKey *)innerkeys->head->data.ptr_value
$22 = {type = T_PathKey, pk_eclass = 0x1a60e08, pk_opfamily = 1994, pk_strategy = 1, pk_nulls_first = false}
(gdb) p *(PathKey *)merge_pathkeys->head->data.ptr_value
$25 = {type = T_PathKey, pk_eclass = 0x1a60e08, pk_opfamily = 1994, pk_strategy = 1, pk_nulls_first = false}
嘗試merge join,進入函式try_mergejoin_path
(gdb)
1038 try_mergejoin_path(root,
(gdb) step
try_mergejoin_path (root=0x1a4dcc0, joinrel=0x1a68e20, outer_path=0x1a62288, inner_path=0x1a62320, pathkeys=0x1a694b8,
mergeclauses=0x1a69518, outersortkeys=0x1a694b8, innersortkeys=0x1a69578, jointype=JOIN_INNER, extra=0x7ffca933f880,
is_partial=false) at joinpath.c:572
572 if (is_partial)
初始merge join成本
...
(gdb)
615 initial_cost_mergejoin(root, &workspace, jointype, mergeclauses,
(gdb) p workspace
$26 = {startup_cost = 10861.483356195882, total_cost = 11134.203356195881, run_cost = 24.997499999999999,
inner_run_cost = 247.72250000000003, inner_rescan_run_cost = 1.3627136827435593e-316, outer_rows = 9999,
inner_rows = 100000, outer_skip_rows = 0, inner_skip_rows = 911, numbuckets = 27665584, numbatches = 0,
inner_rows_total = 1.3681950446447804e-316}
構造merge join
...
(gdb) n
625 create_mergejoin_path(root,
(gdb)
624 add_path(joinrel, (Path *)
(gdb)
644 }
(gdb) p *joinrel->pathlist
$28 = {type = T_List, length = 1, head = 0x1a6a180, tail = 0x1a6a180}
(gdb) p *(Node *)joinrel->pathlist->head->data.ptr_value
$29 = {type = T_MergePath}
(gdb) p *(MergePath *)joinrel->pathlist->head->data.ptr_value
$30 = {jpath = {path = {type = T_MergePath, pathtype = T_MergeJoin, parent = 0x1a68e20, pathtarget = 0x1a69058,
param_info = 0x0, parallel_aware = false, parallel_safe = true, parallel_workers = 0, rows = 100000,
startup_cost = 10863.760856195882, total_cost = 12409.200856195883, pathkeys = 0x1a694b8}, jointype = JOIN_INNER,
inner_unique = false, outerjoinpath = 0x1a62288, innerjoinpath = 0x1a62320, joinrestrictinfo = 0x1a692f8},
path_mergeclauses = 0x1a69518, outersortkeys = 0x1a694b8, innersortkeys = 0x1a69578, skip_mark_restore = false,
materialize_inner = false}
完成呼叫
(gdb) n
sort_inner_and_outer (root=0x1a4dcc0, joinrel=0x1a68e20, outerrel=0x1a4d700, innerrel=0x1a4d918, jointype=JOIN_INNER,
extra=0x7ffca933f880) at joinpath.c:1054
1054 if (cheapest_partial_outer && cheapest_safe_inner)
(gdb)
997 foreach(l, all_pathkeys)
(gdb)
1066 }
(gdb) n
add_paths_to_joinrel (root=0x1a4dcc0, joinrel=0x1a68e20, outerrel=0x1a4d700, innerrel=0x1a4d918, jointype=JOIN_INNER,
sjinfo=0x7ffca933f970, restrictlist=0x1a692f8) at joinpath.c:279
279 if (mergejoin_allowed)
(gdb)
280 match_unsorted_outer(root, joinrel, outerrel, innerrel,
...
DONE!
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-2374834/，如需轉載，請註明出處，否則將追究法律責任。
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