diff options
Diffstat (limited to 'src')
| -rw-r--r-- | src/backend/utils/adt/selfuncs.c | 33 | ||||
| -rw-r--r-- | src/test/regress/expected/subselect.out | 4 |
2 files changed, 27 insertions, 10 deletions
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index c556f4bc6ec..2f2f2c4e5e6 100644 --- a/src/backend/utils/adt/selfuncs.c +++ b/src/backend/utils/adt/selfuncs.c @@ -15,7 +15,7 @@ * * * IDENTIFICATION - * $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.169 2004/12/31 22:01:22 pgsql Exp $ + * $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.169.4.1 2005/01/28 20:35:10 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -1937,10 +1937,13 @@ add_unique_group_var(Query *root, List *varinfos, * if we considered ones of the same rel, we'd be double-counting the * restriction selectivity of the equality in the next step. * 3. For Vars within a single source rel, we multiply together the numbers - * of values, clamp to the number of rows in the rel, and then multiply - * by the selectivity of the restriction clauses for that rel. The - * initial product is probably too high (it's the worst case) but since - * we can clamp to the rel's rows it won't be hugely bad. Multiplying + * of values, clamp to the number of rows in the rel (divided by 10 if + * more than one Var), and then multiply by the selectivity of the + * restriction clauses for that rel. When there's more than one Var, + * the initial product is probably too high (it's the worst case) but + * clamping to a fraction of the rel's rows seems to be a helpful + * heuristic for not letting the estimate get out of hand. (The factor + * of 10 is derived from pre-Postgres-7.4 practice.) Multiplying * by the restriction selectivity is effectively assuming that the * restriction clauses are independent of the grouping, which is a crummy * assumption, but it's hard to do better. @@ -2040,6 +2043,7 @@ estimate_num_groups(Query *root, List *groupExprs, double input_rows) GroupVarInfo *varinfo1 = (GroupVarInfo *) linitial(varinfos); RelOptInfo *rel = varinfo1->rel; double reldistinct = varinfo1->ndistinct; + int relvarcount = 1; List *newvarinfos = NIL; /* @@ -2051,7 +2055,10 @@ estimate_num_groups(Query *root, List *groupExprs, double input_rows) GroupVarInfo *varinfo2 = (GroupVarInfo *) lfirst(l); if (varinfo2->rel == varinfo1->rel) + { reldistinct *= varinfo2->ndistinct; + relvarcount++; + } else { /* not time to process varinfo2 yet */ @@ -2066,10 +2073,20 @@ estimate_num_groups(Query *root, List *groupExprs, double input_rows) if (rel->tuples > 0) { /* - * Clamp to size of rel, multiply by restriction selectivity. + * Clamp to size of rel, or size of rel / 10 if multiple Vars. + * The fudge factor is because the Vars are probably correlated + * but we don't know by how much. + */ + double clamp = rel->tuples; + + if (relvarcount > 1) + clamp *= 0.1; + if (reldistinct > clamp) + reldistinct = clamp; + + /* + * Multiply by restriction selectivity. */ - if (reldistinct > rel->tuples) - reldistinct = rel->tuples; reldistinct *= rel->rows / rel->tuples; /* diff --git a/src/test/regress/expected/subselect.out b/src/test/regress/expected/subselect.out index 07e727de482..56bea035981 100644 --- a/src/test/regress/expected/subselect.out +++ b/src/test/regress/expected/subselect.out @@ -134,11 +134,11 @@ SELECT '' AS five, f1 AS "Correlated Field" WHERE f3 IS NOT NULL); five | Correlated Field ------+------------------ - | 2 | 3 | 1 - | 2 | 3 + | 2 + | 2 (5 rows) -- |