Robert Haas <robertmh...@gmail.com> writes: > On Thu, Feb 16, 2017 at 3:51 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: >> No, it'd be the *most* common MCV, because we're concerned about the >> worst-case (largest) bucket size. But that's good, really, because the >> highest MCV frequency will be the one we have most statistical >> confidence in. There's generally a whole lot of noise in the tail-end >> MCV numbers.

> Oh, right. That's reassuring, as it seems like it has a much better > chance of actually being right. Here's a version that does it that way. Unsurprisingly, it doesn't cause any regression test changes, but you can confirm it's having an effect with this test case: create table tt(f1 int); insert into tt select 1 from generate_series(1,1000000) g; insert into tt select g from generate_series(1,1000000) g; analyze tt; explain select * from tt a natural join tt b; Unpatched code will go for a hash join on this example. For application to the back branches, we could do it just like this (leaving the existing fields alone, and allowing sizeof(RestrictInfo) to grow), or we could change the datatypes of the four fields involved to float4 so that sizeof(RestrictInfo) stays the same. I'm not entirely sure which way is the more hazardous from an ABI standpoint. If there are any external modules doing their own palloc(sizeof(RestrictInfo)) calls, the first way would be bad, but really there shouldn't be; I'd expect people to be using make_restrictinfo() and friends. (Note that palloc's power-of-2 padding wouldn't save us, because sizeof(RestrictInfo) is currently exactly 128 on 32-bit machines in several of the back branches.) Conversely, if any non-core code is touching the bucketsize fields, changing those field widths would break that; but that doesn't seem all that likely either. On balance I think I might go for the first way, because it would remove doubt about whether reducing the precision of the bucketsize estimates would cause any unexpected plan changes. Or we could decide not to back-patch because the problem doesn't come up often enough to justify taking any risk for. But given that we've gotten one confirmed field report, I'm not voting that way. regards, tom lane

diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c index 05d8538..c2cdd79 100644 *** a/src/backend/nodes/copyfuncs.c --- b/src/backend/nodes/copyfuncs.c *************** _copyRestrictInfo(const RestrictInfo *fr *** 2070,2075 **** --- 2070,2077 ---- COPY_SCALAR_FIELD(hashjoinoperator); COPY_SCALAR_FIELD(left_bucketsize); COPY_SCALAR_FIELD(right_bucketsize); + COPY_SCALAR_FIELD(left_mcvfreq); + COPY_SCALAR_FIELD(right_mcvfreq); return newnode; } diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c index d01630f..f88c1c5 100644 *** a/src/backend/optimizer/path/costsize.c --- b/src/backend/optimizer/path/costsize.c *************** final_cost_hashjoin(PlannerInfo *root, H *** 2800,2805 **** --- 2800,2806 ---- double hashjointuples; double virtualbuckets; Selectivity innerbucketsize; + Selectivity innermcvfreq; ListCell *hcl; /* Mark the path with the correct row estimate */ *************** final_cost_hashjoin(PlannerInfo *root, H *** 2827,2835 **** virtualbuckets = (double) numbuckets *(double) numbatches; /* ! * Determine bucketsize fraction for inner relation. We use the smallest ! * bucketsize estimated for any individual hashclause; this is undoubtedly ! * conservative. * * BUT: if inner relation has been unique-ified, we can assume it's good * for hashing. This is important both because it's the right answer, and --- 2828,2836 ---- virtualbuckets = (double) numbuckets *(double) numbatches; /* ! * Determine bucketsize fraction and MCV frequency for the inner relation. ! * We use the smallest bucketsize or MCV frequency estimated for any ! * individual hashclause; this is undoubtedly conservative. * * BUT: if inner relation has been unique-ified, we can assume it's good * for hashing. This is important both because it's the right answer, and *************** final_cost_hashjoin(PlannerInfo *root, H *** 2837,2850 **** --- 2838,2856 ---- * non-unique-ified paths. */ if (IsA(inner_path, UniquePath)) + { innerbucketsize = 1.0 / virtualbuckets; + innermcvfreq = 0.0; + } else { innerbucketsize = 1.0; + innermcvfreq = 1.0; foreach(hcl, hashclauses) { RestrictInfo *restrictinfo = (RestrictInfo *) lfirst(hcl); Selectivity thisbucketsize; + Selectivity thismcvfreq; Assert(IsA(restrictinfo, RestrictInfo)); *************** final_cost_hashjoin(PlannerInfo *root, H *** 2853,2860 **** * is the inner side. * * Since we tend to visit the same clauses over and over when ! * planning a large query, we cache the bucketsize estimate in the ! * RestrictInfo node to avoid repeated lookups of statistics. */ if (bms_is_subset(restrictinfo->right_relids, inner_path->parent->relids)) --- 2859,2866 ---- * is the inner side. * * Since we tend to visit the same clauses over and over when ! * planning a large query, we cache the bucket stats estimates in ! * the RestrictInfo node to avoid repeated lookups of statistics. */ if (bms_is_subset(restrictinfo->right_relids, inner_path->parent->relids)) *************** final_cost_hashjoin(PlannerInfo *root, H *** 2864,2875 **** if (thisbucketsize < 0) { /* not cached yet */ ! thisbucketsize = ! estimate_hash_bucketsize(root, get_rightop(restrictinfo->clause), ! virtualbuckets); restrictinfo->right_bucketsize = thisbucketsize; } } else { --- 2870,2884 ---- if (thisbucketsize < 0) { /* not cached yet */ ! estimate_hash_bucket_stats(root, get_rightop(restrictinfo->clause), ! virtualbuckets, ! &thismcvfreq, ! &thisbucketsize); restrictinfo->right_bucketsize = thisbucketsize; + restrictinfo->right_mcvfreq = thismcvfreq; } + thismcvfreq = restrictinfo->right_mcvfreq; } else { *************** final_cost_hashjoin(PlannerInfo *root, H *** 2880,2899 **** if (thisbucketsize < 0) { /* not cached yet */ ! thisbucketsize = ! estimate_hash_bucketsize(root, get_leftop(restrictinfo->clause), ! virtualbuckets); restrictinfo->left_bucketsize = thisbucketsize; } } if (innerbucketsize > thisbucketsize) innerbucketsize = thisbucketsize; } } /* * Compute cost of the hashquals and qpquals (other restriction clauses) * separately. */ --- 2889,2926 ---- if (thisbucketsize < 0) { /* not cached yet */ ! estimate_hash_bucket_stats(root, get_leftop(restrictinfo->clause), ! virtualbuckets, ! &thismcvfreq, ! &thisbucketsize); restrictinfo->left_bucketsize = thisbucketsize; + restrictinfo->left_mcvfreq = thismcvfreq; } + thismcvfreq = restrictinfo->left_mcvfreq; } if (innerbucketsize > thisbucketsize) innerbucketsize = thisbucketsize; + if (innermcvfreq > thismcvfreq) + innermcvfreq = thismcvfreq; } } /* + * If the bucket holding the inner MCV would exceed work_mem, we don't + * want to hash unless there is really no other alternative, so apply + * disable_cost. (The executor normally copes with excessive memory usage + * by splitting batches, but obviously it cannot separate equal values + * that way, so it will be unable to drive the batch size below work_mem + * when this is true.) + */ + if (relation_byte_size(clamp_row_est(inner_path_rows * innermcvfreq), + inner_path->pathtarget->width) > + (work_mem * 1024L)) + startup_cost += disable_cost; + + /* * Compute cost of the hashquals and qpquals (other restriction clauses) * separately. */ diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c index 06e843d..f6ddb2e 100644 *** a/src/backend/optimizer/prep/prepunion.c --- b/src/backend/optimizer/prep/prepunion.c *************** adjust_appendrel_attrs_mutator(Node *nod *** 1961,1966 **** --- 1961,1968 ---- newinfo->scansel_cache = NIL; newinfo->left_bucketsize = -1; newinfo->right_bucketsize = -1; + newinfo->left_mcvfreq = -1; + newinfo->right_mcvfreq = -1; return (Node *) newinfo; } diff --git a/src/backend/optimizer/util/restrictinfo.c b/src/backend/optimizer/util/restrictinfo.c index 045b5cf..e7efe36 100644 *** a/src/backend/optimizer/util/restrictinfo.c --- b/src/backend/optimizer/util/restrictinfo.c *************** make_restrictinfo_internal(Expr *clause, *** 199,204 **** --- 199,206 ---- restrictinfo->left_bucketsize = -1; restrictinfo->right_bucketsize = -1; + restrictinfo->left_mcvfreq = -1; + restrictinfo->right_mcvfreq = -1; return restrictinfo; } diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index d14f0f9..816cb65 100644 *** a/src/backend/utils/adt/selfuncs.c --- b/src/backend/utils/adt/selfuncs.c *************** estimate_num_groups(PlannerInfo *root, L *** 3521,3529 **** } /* ! * Estimate hash bucketsize fraction (ie, number of entries in a bucket ! * divided by total tuples in relation) if the specified expression 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 --- 3521,3536 ---- } /* ! * Estimate hash bucket statistics when the specified expression is used ! * as a hash key for the given number of buckets. ! * ! * This attempts to determine two values: ! * ! * 1. The frequency of the most common value of the expression (returns ! * zero into *mcv_freq if we can't get that). ! * ! * 2. The "bucketsize fraction", ie, average number of entries in a bucket ! * divided by total tuples in relation. * * XXX This is really pretty bogus since we're effectively assuming that the * distribution of hash keys will be the same after applying restriction *************** estimate_num_groups(PlannerInfo *root, L *** 3549,3563 **** * 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). */ ! Selectivity ! estimate_hash_bucketsize(PlannerInfo *root, Node *hashkey, double nbuckets) { VariableStatData vardata; double estfract, ndistinct, stanullfrac, - mcvfreq, avgfreq; bool isdefault; float4 *numbers; --- 3556,3577 ---- * 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). + * + * The caller should also check that the mcv_freq is not so large that the + * most common value would by itself require an impractically large bucket. + * In a hash join, the executor can split buckets if they get too big, but + * obviously that doesn't help for a bucket that contains many duplicates of + * the same value. */ ! void ! estimate_hash_bucket_stats(PlannerInfo *root, Node *hashkey, double nbuckets, ! Selectivity *mcv_freq, ! Selectivity *bucketsize_frac) { VariableStatData vardata; double estfract, ndistinct, stanullfrac, avgfreq; bool isdefault; float4 *numbers; *************** estimate_hash_bucketsize(PlannerInfo *ro *** 3565,3578 **** examine_variable(root, hashkey, 0, &vardata); /* Get number of distinct values */ ndistinct = get_variable_numdistinct(&vardata, &isdefault); ! /* If ndistinct isn't real, punt and return 0.1, per comments above */ if (isdefault) { ReleaseVariableStats(vardata); ! return (Selectivity) 0.1; } /* Get fraction that are null */ --- 3579,3618 ---- examine_variable(root, hashkey, 0, &vardata); + /* Look up the frequency of the most common value, if available */ + *mcv_freq = 0.0; + + if (HeapTupleIsValid(vardata.statsTuple)) + { + if (get_attstatsslot(vardata.statsTuple, + vardata.atttype, vardata.atttypmod, + STATISTIC_KIND_MCV, InvalidOid, + NULL, + NULL, NULL, + &numbers, &nnumbers)) + { + /* + * The first MCV stat is for the most common value. + */ + if (nnumbers > 0) + *mcv_freq = numbers[0]; + free_attstatsslot(vardata.atttype, NULL, 0, + numbers, nnumbers); + } + } + /* Get number of distinct values */ ndistinct = get_variable_numdistinct(&vardata, &isdefault); ! /* ! * If ndistinct isn't real, punt. We normally return 0.1, but if the ! * mcv_freq is known to be even higher than that, use it instead. ! */ if (isdefault) { + *bucketsize_frac = (Selectivity) Max(0.1, *mcv_freq); ReleaseVariableStats(vardata); ! return; } /* Get fraction that are null */ *************** estimate_hash_bucketsize(PlannerInfo *ro *** 3614,3647 **** estfract = 1.0 / ndistinct; /* - * Look up the frequency of the most common value, if available. - */ - mcvfreq = 0.0; - - if (HeapTupleIsValid(vardata.statsTuple)) - { - if (get_attstatsslot(vardata.statsTuple, - vardata.atttype, vardata.atttypmod, - STATISTIC_KIND_MCV, InvalidOid, - NULL, - NULL, NULL, - &numbers, &nnumbers)) - { - /* - * The first MCV stat is for the most common value. - */ - if (nnumbers > 0) - mcvfreq = numbers[0]; - free_attstatsslot(vardata.atttype, 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 --- 3654,3663 ---- estfract = 1.0 / ndistinct; /* * Adjust estimated bucketsize upward to account for skewed distribution. */ ! if (avgfreq > 0.0 && *mcv_freq > avgfreq) ! estfract *= *mcv_freq / avgfreq; /* * Clamp bucketsize to sane range (the above adjustment could easily *************** estimate_hash_bucketsize(PlannerInfo *ro *** 3653,3661 **** else if (estfract > 1.0) estfract = 1.0; ! ReleaseVariableStats(vardata); ! return (Selectivity) estfract; } --- 3669,3677 ---- else if (estfract > 1.0) estfract = 1.0; ! *bucketsize_frac = (Selectivity) estfract; ! ReleaseVariableStats(vardata); } diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h index f7ac6f6..2b2d950 100644 *** a/src/include/nodes/relation.h --- b/src/include/nodes/relation.h *************** typedef struct RestrictInfo *** 1697,1702 **** --- 1697,1704 ---- /* cache space for hashclause processing; -1 if not yet set */ Selectivity left_bucketsize; /* avg bucketsize of left side */ Selectivity right_bucketsize; /* avg bucketsize of right side */ + Selectivity left_mcvfreq; /* most common val's freq on left side */ + Selectivity right_mcvfreq; /* most common val's freq on right side */ } RestrictInfo; /* diff --git a/src/include/utils/selfuncs.h b/src/include/utils/selfuncs.h index 9f9d2dc..03ed27b 100644 *** a/src/include/utils/selfuncs.h --- b/src/include/utils/selfuncs.h *************** extern void mergejoinscansel(PlannerInfo *** 204,211 **** extern double estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows, List **pgset); ! extern Selectivity estimate_hash_bucketsize(PlannerInfo *root, Node *hashkey, ! double nbuckets); extern List *deconstruct_indexquals(IndexPath *path); extern void genericcostestimate(PlannerInfo *root, IndexPath *path, --- 204,213 ---- extern double estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows, List **pgset); ! extern void estimate_hash_bucket_stats(PlannerInfo *root, ! Node *hashkey, double nbuckets, ! Selectivity *mcv_freq, ! Selectivity *bucketsize_frac); extern List *deconstruct_indexquals(IndexPath *path); extern void genericcostestimate(PlannerInfo *root, IndexPath *path,

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