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Add hash_mem_multiplier GUC.

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Add a GUC that acts as a multiplier on work_mem.  It gets applied when
sizing executor node hash tables that were previously size constrained
using work_mem alone.

The new GUC can be used to preferentially give hash-based nodes more
memory than the generic work_mem limit.  It is intended to enable admin
tuning of the executor's memory usage.  Overall system throughput and
system responsiveness can be improved by giving hash-based executor
nodes more memory (especially over sort-based alternatives, which are
often much less sensitive to being memory constrained).

The default value for hash_mem_multiplier is 1.0, which is also the
minimum valid value.  This means that hash-based nodes continue to apply
work_mem in the traditional way by default.

hash_mem_multiplier is generally useful.  However, it is being added now
due to concerns about hash aggregate performance stability for users
that upgrade to Postgres 13 (which added disk-based hash aggregation in
commit 1f39bce0).  While the old hash aggregate behavior risked
out-of-memory errors, it is nevertheless likely that many users actually
benefited.  Hash agg's previous indifference to work_mem during query
execution was not just faster; it also accidentally made aggregation
resilient to grouping estimate problems (at least in cases where this
didn't create destabilizing memory pressure).

hash_mem_multiplier can provide a certain kind of continuity with the
behavior of Postgres 12 hash aggregates in cases where the planner
incorrectly estimates that all groups (plus related allocations) will
fit in work_mem/hash_mem.  This seems necessary because hash-based
aggregation is usually much slower when only a small fraction of all
groups can fit.  Even when it isn't possible to totally avoid hash
aggregates that spill, giving hash aggregation more memory will reliably
improve performance (the same cannot be said for external sort
operations, which appear to be almost unaffected by memory availability
provided it's at least possible to get a single merge pass).

The PostgreSQL 13 release notes should advise users that increasing
hash_mem_multiplier can help with performance regressions associated
with hash aggregation.  That can be taken care of by a later commit.

Author: Peter Geoghegan
Reviewed-By: Álvaro Herrera, Jeff Davis
Discussion: https://postgr.es/m/20200625203629.7m6yvut7eqblgmfo@alap3.anarazel.de
Discussion: https://postgr.es/m/CAH2-WzmD%2Bi1pG6rc1%2BCjc4V6EaFJ_qSuKCCHVnH%3DoruqD-zqow%40mail.gmail.com
Backpatch: 13-, where disk-based hash aggregation was introduced.
This commit is contained in:
Peter Geoghegan
2020-07-29 14:14:58 -07:00
parent 6023b7ea71
commit d6c08e29e7
19 changed files with 205 additions and 81 deletions
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12
src/backend/optimizer/path/costsize.c
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@@ -3525,7 +3525,7 @@ initial_cost_hashjoin(PlannerInfo *root, JoinCostWorkspace *workspace,
* Get hash table size that executor would use for inner relation.
*
* XXX for the moment, always assume that skew optimization will be
* performed. As long as SKEW_WORK_MEM_PERCENT is small, it's not worth
* performed. As long as SKEW_HASH_MEM_PERCENT is small, it's not worth
* trying to determine that for sure.
*
* XXX at some point it might be interesting to try to account for skew
@@ -3534,7 +3534,7 @@ initial_cost_hashjoin(PlannerInfo *root, JoinCostWorkspace *workspace,
ExecChooseHashTableSize(inner_path_rows_total,
inner_path->pathtarget->width,
true, /* useskew */
parallel_hash, /* try_combined_work_mem */
parallel_hash, /* try_combined_hash_mem */
outer_path->parallel_workers,
&space_allowed,
&numbuckets,
@@ -3597,6 +3597,7 @@ final_cost_hashjoin(PlannerInfo *root, HashPath *path,
Cost run_cost = workspace->run_cost;
int numbuckets = workspace->numbuckets;
int numbatches = workspace->numbatches;
int hash_mem;
Cost cpu_per_tuple;
QualCost hash_qual_cost;
QualCost qp_qual_cost;
@@ -3715,16 +3716,17 @@ final_cost_hashjoin(PlannerInfo *root, HashPath *path,
}
/*
* If the bucket holding the inner MCV would exceed work_mem, we don't
* If the bucket holding the inner MCV would exceed hash_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
* that way, so it will be unable to drive the batch size below hash_mem
* when this is true.)
*/
hash_mem = get_hash_mem();
if (relation_byte_size(clamp_row_est(inner_path_rows * innermcvfreq),
inner_path->pathtarget->width) >
(work_mem * 1024L))
(hash_mem * 1024L))
startup_cost += disable_cost;
/*