diff options
| author | Tom Lane | 2000-04-18 05:43:02 +0000 |
|---|---|---|
| committer | Tom Lane | 2000-04-18 05:43:02 +0000 |
| commit | 25442d8d2fd35389813062f523488821f4fc31d4 (patch) | |
| tree | d58f9ec0b5a7d3074c222008a9d58d5c35db5251 /src/backend/optimizer | |
| parent | 24864d048eec2c579346eb31a42c87be1c92644e (diff) | |
Correct oversight in hashjoin cost estimation: nodeHash sizes its hash
table for an average of NTUP_PER_BUCKET tuples/bucket, but cost_hashjoin
was assuming a target load of one tuple/bucket. This was causing a
noticeable underestimate of hashjoin costs.
Diffstat (limited to 'src/backend/optimizer')
| -rw-r--r-- | src/backend/optimizer/path/costsize.c | 16 |
1 files changed, 11 insertions, 5 deletions
diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c index 6ecfb2a4713..df3c6d5c429 100644 --- a/src/backend/optimizer/path/costsize.c +++ b/src/backend/optimizer/path/costsize.c @@ -42,7 +42,7 @@ * Portions Copyright (c) 1994, Regents of the University of California * * IDENTIFICATION - * $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.57 2000/04/12 17:15:19 momjian Exp $ + * $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.58 2000/04/18 05:43:02 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -51,6 +51,7 @@ #include <math.h> +#include "executor/nodeHash.h" #include "miscadmin.h" #include "nodes/plannodes.h" #include "optimizer/clauses.h" @@ -604,12 +605,17 @@ cost_hashjoin(Path *path, run_cost += cpu_operator_cost * outer_path->parent->rows; /* - * the number of tuple comparisons needed is the number of outer - * tuples times the typical hash bucket size, which we estimate - * conservatively as the inner disbursion times the inner tuple count. + * The number of tuple comparisons needed is the number of outer + * tuples times the typical hash bucket size. nodeHash.c tries for + * average bucket loading of NTUP_PER_BUCKET, but that goal will + * be reached only if data values are uniformly distributed among + * the buckets. To be conservative, we scale up the target bucket + * size by the number of inner rows times inner disbursion, giving + * an estimate of the typical number of duplicates of each value. + * We then charge one cpu_operator_cost per tuple comparison. */ run_cost += cpu_operator_cost * outer_path->parent->rows * - ceil(inner_path->parent->rows * innerdisbursion); + NTUP_PER_BUCKET * ceil(inner_path->parent->rows * innerdisbursion); /* * Estimate the number of tuples that get through the hashing filter |