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Fix hash partition pruning with asymmetric partition sets.

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perform_pruning_combine_step() was not taught about the number of
partition indexes used in hash partitioning; more embarrassingly,
get_matching_hash_bounds() also had it wrong.  These errors are masked
in the common case where all the partitions have the same modulus
and no partition is missing.  However, with missing or unequal-size
partitions, we could erroneously prune some partitions that need
to be scanned, leading to silently wrong query answers.

While a minimal-footprint fix for this could be to export
get_partition_bound_num_indexes and make the incorrect functions use it,
I'm of the opinion that that function should never have existed in the
first place.  It's not reasonable data structure design that
PartitionBoundInfoData lacks any explicit record of the length of
its indexes[] array.  Perhaps that was all right when it could always
be assumed equal to ndatums, but something should have been done about
it as soon as that stopped being true.  Putting in an explicit
"nindexes" field makes both partition_bounds_equal() and
partition_bounds_copy() simpler, safer, and faster than before,
and removes explicit knowledge of the number-of-partition-indexes
rules from some other places too.

This change also makes get_hash_partition_greatest_modulus obsolete.
I left that in place in case any external code uses it, but no core
code does anymore.

Per bug #16840 from Michał Albrycht.  Back-patch to v11 where the
hash partitioning code came in.  (In the back branches, add the new
field at the end of PartitionBoundInfoData to minimize ABI risks.)

Discussion: https://postgr.es/m/16840-571a22976f829ad4@postgresql.org
This commit is contained in:
Tom Lane
2021-01-28 13:41:55 -05:00
parent 1b242f42ba
commit 1d9351a87c
6 changed files with 125 additions and 142 deletions
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4
src/backend/executor/execPartition.c
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@ -1323,16 +1323,14 @@ get_partition_for_tuple(PartitionDispatch pd, Datum *values, bool *isnull)
{
case PARTITION_STRATEGY_HASH:
{
int greatest_modulus;
uint64 rowHash;
greatest_modulus = get_hash_partition_greatest_modulus(boundinfo);
rowHash = compute_partition_hash_value(key->partnatts,
key->partsupfunc,
key->partcollation,
values, isnull);
part_index = boundinfo->indexes[rowHash % greatest_modulus];
part_index = boundinfo->indexes[rowHash % boundinfo->nindexes];
}
break;