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| author | Alvaro Herrera <alvherre@alvh.no-ip.org> | 2017-03-27 12:52:50 -0300 |
|---|---|---|
| committer | Alvaro Herrera <alvherre@alvh.no-ip.org> | 2017-03-27 13:14:23 -0300 |
| commit | 1f171a1803c28d3ae24636c9ca3352ec82c39e5f (patch) | |
| tree | 48fd0576fb3248f2cd6e740c8d7c0107fc39686a /src | |
| parent | 827d6f977940952ebef4bd21fb0f97be4e20c0c4 (diff) | |
| download | postgresql-1f171a1803c28d3ae24636c9ca3352ec82c39e5f.tar.gz postgresql-1f171a1803c28d3ae24636c9ca3352ec82c39e5f.zip | |
Fix thinko in estimate_num_groups
The code for the reworked n-distinct estimation on commit 7b504eb282 was
written differently in a previous version of the patch, prior to commit;
on rewriting it, we missed updating an initializer. This caused the
code to (mistakenly) apply a fudge factor even in the case where a
single value is applied, leading to incorrect results.
This means that the 'relvarcount' variable name is now wrong. Add a
comment to try and make the situation clearer, and remove an incorrect
comment I added.
Problem noticed, and code patch, by Tomas Vondra. Additional commentary
by Álvaro.
Diffstat (limited to 'src')
| -rw-r--r-- | src/backend/utils/adt/selfuncs.c | 8 |
1 files changed, 6 insertions, 2 deletions
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index cc24c8aeb56..5c382a2013e 100644 --- a/src/backend/utils/adt/selfuncs.c +++ b/src/backend/utils/adt/selfuncs.c @@ -3404,7 +3404,7 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows, RelOptInfo *rel = varinfo1->rel; double reldistinct = 1; double relmaxndistinct = reldistinct; - int relvarcount = 1; + int relvarcount = 0; List *newvarinfos = NIL; List *relvarinfos = NIL; @@ -3436,6 +3436,10 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows, * we multiply them together. Any remaining relvarinfos after * no more multivariate matches are found are assumed independent too, * so their individual ndistinct estimates are multiplied also. + * + * While iterating, count how many separate numdistinct values we + * apply. We apply a fudge factor below, but only if we multiplied + * more than one such values. */ while (relvarinfos) { @@ -3447,7 +3451,7 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows, reldistinct *= mvndistinct; if (relmaxndistinct < mvndistinct) relmaxndistinct = mvndistinct; - relvarcount++; /* inaccurate, but doesn't matter */ + relvarcount++; } else { |