Movatterモバイル変換

[0]ホーム
URL:

Skip to content

Navigation Menu

Sign in
Appearance settings

Search code, repositories, users, issues, pull requests...

Provide feedback

We read every piece of feedback, and take your input very seriously.

Saved searches

Use saved searches to filter your results more quickly

 Sign up
Appearance settings

Commitca5f885

Browse files
committed
Improve eqjoinsel_semi's behavior for small inner relations with no stats.
If we don't have any MCV statistics for the inner relation, and we don'ttrust its numdistinct estimate either, eqjoinsel_semi falls back to a veryconservative estimate (that 50% of the outer rows have matches). This isparticularly problematic if the inner relation is completely empty, sincethen even an explicit ANALYZE won't produce any pg_statistic entries,so there's no way to budge the planner off the bad estimate.We'd produce a better estimate in such cases if we used the nd2/nd1selectivity heuristic, so an easy fix is to treat the nd2 estimate asnon-default if we derive it from clamping to the inner rel's rowcountestimate. This won't fix every related case (mainly because the rowcountestimate might be larger than DEFAULT_NUM_DISTINCT), but it seems like asane extension of the existing logic, so let's apply the change in HEADand see if anyone complains. Per bug #14438 from Nikolay Nikitin.Report:https://postgr.es/m/20161128182113.6527.58926@wrigleys.postgresql.orgDiscussion:https://postgr.es/m/31089.1480384713@sss.pgh.pa.us
1 parent96fb4c9 commitca5f885

File tree

1 file changed

+16
-2
lines changed

1 file changed

+16
-2
lines changed

‎src/backend/utils/adt/selfuncs.c

Lines changed: 16 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -2511,10 +2511,24 @@ eqjoinsel_semi(Oid operator,
25112511
* We can apply this clamping both with respect to the base relation from
25122512
* which the join variable comes (if there is just one), and to the
25132513
* immediate inner input relation of the current join.
2514+
*
2515+
* If we clamp, we can treat nd2 as being a non-default estimate; it's not
2516+
* great, maybe, but it didn't come out of nowhere either. This is most
2517+
* helpful when the inner relation is empty and consequently has no stats.
25142518
*/
25152519
if (vardata2->rel)
2516-
nd2=Min(nd2,vardata2->rel->rows);
2517-
nd2=Min(nd2,inner_rel->rows);
2520+
{
2521+
if (nd2 >=vardata2->rel->rows)
2522+
{
2523+
nd2=vardata2->rel->rows;
2524+
isdefault2= false;
2525+
}
2526+
}
2527+
if (nd2 >=inner_rel->rows)
2528+
{
2529+
nd2=inner_rel->rows;
2530+
isdefault2= false;
2531+
}
25182532

25192533
if (HeapTupleIsValid(vardata1->statsTuple))
25202534
{

0 commit comments

Comments
 (0)
[8]ページ先頭
©2009-2025 Movatter.jp