diff options
Diffstat (limited to 'src/backend/optimizer')
| -rw-r--r-- | src/backend/optimizer/path/costsize.c | 181 | ||||
| -rw-r--r-- | src/backend/optimizer/util/relnode.c | 8 |
2 files changed, 5 insertions, 184 deletions
diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c index c23cf4d2324..79674ac4b94 100644 --- a/src/backend/optimizer/path/costsize.c +++ b/src/backend/optimizer/path/costsize.c @@ -49,7 +49,7 @@ * Portions Copyright (c) 1994, Regents of the University of California * * IDENTIFICATION - * $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.124 2004/02/03 17:34:03 tgl Exp $ + * $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.125 2004/02/17 00:52:53 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -102,8 +102,6 @@ bool enable_mergejoin = true; bool enable_hashjoin = true; -static Selectivity estimate_hash_bucketsize(Query *root, Var *var, - int nbuckets); static bool cost_qual_eval_walker(Node *node, QualCost *total); static Selectivity approx_selectivity(Query *root, List *quals, JoinType jointype); @@ -1152,7 +1150,7 @@ cost_hashjoin(HashPath *path, Query *root) /* not cached yet */ thisbucketsize = estimate_hash_bucketsize(root, - (Var *) get_rightop(restrictinfo->clause), + get_rightop(restrictinfo->clause), virtualbuckets); restrictinfo->right_bucketsize = thisbucketsize; } @@ -1168,7 +1166,7 @@ cost_hashjoin(HashPath *path, Query *root) /* not cached yet */ thisbucketsize = estimate_hash_bucketsize(root, - (Var *) get_leftop(restrictinfo->clause), + get_leftop(restrictinfo->clause), virtualbuckets); restrictinfo->left_bucketsize = thisbucketsize; } @@ -1249,179 +1247,6 @@ cost_hashjoin(HashPath *path, Query *root) path->jpath.path.total_cost = startup_cost + run_cost; } -/* - * Estimate hash bucketsize fraction (ie, number of entries in a bucket - * divided by total tuples in relation) if the specified Var is used - * as a hash key. - * - * XXX This is really pretty bogus since we're effectively assuming that the - * distribution of hash keys will be the same after applying restriction - * clauses as it was in the underlying relation. However, we are not nearly - * smart enough to figure out how the restrict clauses might change the - * distribution, so this will have to do for now. - * - * We are passed the number of buckets the executor will use for the given - * input relation. If the data were perfectly distributed, with the same - * number of tuples going into each available bucket, then the bucketsize - * fraction would be 1/nbuckets. But this happy state of affairs will occur - * only if (a) there are at least nbuckets distinct data values, and (b) - * we have a not-too-skewed data distribution. Otherwise the buckets will - * be nonuniformly occupied. If the other relation in the join has a key - * distribution similar to this one's, then the most-loaded buckets are - * exactly those that will be probed most often. Therefore, the "average" - * bucket size for costing purposes should really be taken as something close - * to the "worst case" bucket size. We try to estimate this by adjusting the - * fraction if there are too few distinct data values, and then scaling up - * by the ratio of the most common value's frequency to the average frequency. - * - * If no statistics are available, use a default estimate of 0.1. This will - * discourage use of a hash rather strongly if the inner relation is large, - * which is what we want. We do not want to hash unless we know that the - * inner rel is well-dispersed (or the alternatives seem much worse). - */ -static Selectivity -estimate_hash_bucketsize(Query *root, Var *var, int nbuckets) -{ - Oid relid; - RelOptInfo *rel; - HeapTuple tuple; - Form_pg_statistic stats; - double estfract, - ndistinct, - mcvfreq, - avgfreq; - float4 *numbers; - int nnumbers; - - /* Ignore any binary-compatible relabeling */ - if (var && IsA(var, RelabelType)) - var = (Var *) ((RelabelType *) var)->arg; - - /* - * Lookup info about var's relation and attribute; if none available, - * return default estimate. - */ - if (var == NULL || !IsA(var, Var)) - return 0.1; - - relid = getrelid(var->varno, root->rtable); - if (relid == InvalidOid) - return 0.1; - - rel = find_base_rel(root, var->varno); - - if (rel->tuples <= 0.0 || rel->rows <= 0.0) - return 0.1; /* ensure we can divide below */ - - tuple = SearchSysCache(STATRELATT, - ObjectIdGetDatum(relid), - Int16GetDatum(var->varattno), - 0, 0); - if (!HeapTupleIsValid(tuple)) - { - /* - * If the attribute is known unique because of an index, - * we can treat it as well-distributed. - */ - if (has_unique_index(rel, var->varattno)) - return 1.0 / (double) nbuckets; - - /* - * Perhaps the Var is a system attribute; if so, it will have no - * entry in pg_statistic, but we may be able to guess something - * about its distribution anyway. - */ - switch (var->varattno) - { - case ObjectIdAttributeNumber: - case SelfItemPointerAttributeNumber: - /* these are unique, so buckets should be well-distributed */ - return 1.0 / (double) nbuckets; - case TableOidAttributeNumber: - /* hashing this is a terrible idea... */ - return 1.0; - } - return 0.1; - } - stats = (Form_pg_statistic) GETSTRUCT(tuple); - - /* - * Obtain number of distinct data values in raw relation. - */ - ndistinct = stats->stadistinct; - if (ndistinct < 0.0) - ndistinct = -ndistinct * rel->tuples; - - if (ndistinct <= 0.0) /* ensure we can divide */ - { - ReleaseSysCache(tuple); - return 0.1; - } - - /* Also compute avg freq of all distinct data values in raw relation */ - avgfreq = (1.0 - stats->stanullfrac) / ndistinct; - - /* - * Adjust ndistinct to account for restriction clauses. Observe we - * are assuming that the data distribution is affected uniformly by - * the restriction clauses! - * - * XXX Possibly better way, but much more expensive: multiply by - * selectivity of rel's restriction clauses that mention the target - * Var. - */ - ndistinct *= rel->rows / rel->tuples; - - /* - * Initial estimate of bucketsize fraction is 1/nbuckets as long as - * the number of buckets is less than the expected number of distinct - * values; otherwise it is 1/ndistinct. - */ - if (ndistinct > (double) nbuckets) - estfract = 1.0 / (double) nbuckets; - else - estfract = 1.0 / ndistinct; - - /* - * Look up the frequency of the most common value, if available. - */ - mcvfreq = 0.0; - - if (get_attstatsslot(tuple, var->vartype, var->vartypmod, - STATISTIC_KIND_MCV, InvalidOid, - NULL, NULL, &numbers, &nnumbers)) - { - /* - * The first MCV stat is for the most common value. - */ - if (nnumbers > 0) - mcvfreq = numbers[0]; - free_attstatsslot(var->vartype, NULL, 0, - numbers, nnumbers); - } - - /* - * Adjust estimated bucketsize upward to account for skewed - * distribution. - */ - if (avgfreq > 0.0 && mcvfreq > avgfreq) - estfract *= mcvfreq / avgfreq; - - /* - * Clamp bucketsize to sane range (the above adjustment could easily - * produce an out-of-range result). We set the lower bound a little - * above zero, since zero isn't a very sane result. - */ - if (estfract < 1.0e-6) - estfract = 1.0e-6; - else if (estfract > 1.0) - estfract = 1.0; - - ReleaseSysCache(tuple); - - return (Selectivity) estfract; -} - /* * cost_qual_eval diff --git a/src/backend/optimizer/util/relnode.c b/src/backend/optimizer/util/relnode.c index d6d093ea467..d5a5480c62e 100644 --- a/src/backend/optimizer/util/relnode.c +++ b/src/backend/optimizer/util/relnode.c @@ -8,7 +8,7 @@ * * * IDENTIFICATION - * $PostgreSQL: pgsql/src/backend/optimizer/util/relnode.c,v 1.54 2003/12/08 18:19:58 tgl Exp $ + * $PostgreSQL: pgsql/src/backend/optimizer/util/relnode.c,v 1.55 2004/02/17 00:52:53 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -214,12 +214,8 @@ find_base_rel(Query *root, int relid) * find_join_rel * Returns relation entry corresponding to 'relids' (a set of RT indexes), * or NULL if none exists. This is for join relations. - * - * Note: there is probably no good reason for this to be called from - * anywhere except build_join_rel, but keep it as a separate routine - * just in case. */ -static RelOptInfo * +RelOptInfo * find_join_rel(Query *root, Relids relids) { List *joinrels; |