diff options
| author | Andres Freund | 2017-12-21 07:39:21 +0000 |
|---|---|---|
| committer | Andres Freund | 2017-12-21 08:43:41 +0000 |
| commit | 1804284042e659e7d16904e7bbb0ad546394b6a3 (patch) | |
| tree | d1980f7d94cb31aed8880007e5a41ede08d0cb84 /src/include/optimizer | |
| parent | f94eec490b2671399c102b89c9fa0311aea3a39f (diff) | |
Add parallel-aware hash joins.
Introduce parallel-aware hash joins that appear in EXPLAIN plans as Parallel
Hash Join with Parallel Hash. While hash joins could already appear in
parallel queries, they were previously always parallel-oblivious and had a
partial subplan only on the outer side, meaning that the work of the inner
subplan was duplicated in every worker.
After this commit, the planner will consider using a partial subplan on the
inner side too, using the Parallel Hash node to divide the work over the
available CPU cores and combine its results in shared memory. If the join
needs to be split into multiple batches in order to respect work_mem, then
workers process different batches as much as possible and then work together
on the remaining batches.
The advantages of a parallel-aware hash join over a parallel-oblivious hash
join used in a parallel query are that it:
* avoids wasting memory on duplicated hash tables
* avoids wasting disk space on duplicated batch files
* divides the work of building the hash table over the CPUs
One disadvantage is that there is some communication between the participating
CPUs which might outweigh the benefits of parallelism in the case of small
hash tables. This is avoided by the planner's existing reluctance to supply
partial plans for small scans, but it may be necessary to estimate
synchronization costs in future if that situation changes. Another is that
outer batch 0 must be written to disk if multiple batches are required.
A potential future advantage of parallel-aware hash joins is that right and
full outer joins could be supported, since there is a single set of matched
bits for each hashtable, but that is not yet implemented.
A new GUC enable_parallel_hash is defined to control the feature, defaulting
to on.
Author: Thomas Munro
Reviewed-By: Andres Freund, Robert Haas
Tested-By: Rafia Sabih, Prabhat Sahu
Discussion:
https://postgr.es/m/CAEepm=2W=cOkiZxcg6qiFQP-dHUe09aqTrEMM7yJDrHMhDv_RA@mail.gmail.com
https://postgr.es/m/CAEepm=37HKyJ4U6XOLi=JgfSHM3o6B-GaeO-6hkOmneTDkH+Uw@mail.gmail.com
Diffstat (limited to 'src/include/optimizer')
| -rw-r--r-- | src/include/optimizer/cost.h | 4 | ||||
| -rw-r--r-- | src/include/optimizer/pathnode.h | 1 |
2 files changed, 4 insertions, 1 deletions
diff --git a/src/include/optimizer/cost.h b/src/include/optimizer/cost.h index 5a1fbf97c38..27afc2eaeb3 100644 --- a/src/include/optimizer/cost.h +++ b/src/include/optimizer/cost.h @@ -69,6 +69,7 @@ extern bool enable_hashjoin; extern bool enable_gathermerge; extern bool enable_partition_wise_join; extern bool enable_parallel_append; +extern bool enable_parallel_hash; extern int constraint_exclusion; extern double clamp_row_est(double nrows); @@ -153,7 +154,8 @@ extern void initial_cost_hashjoin(PlannerInfo *root, JoinType jointype, List *hashclauses, Path *outer_path, Path *inner_path, - JoinPathExtraData *extra); + JoinPathExtraData *extra, + bool parallel_hash); extern void final_cost_hashjoin(PlannerInfo *root, HashPath *path, JoinCostWorkspace *workspace, JoinPathExtraData *extra); diff --git a/src/include/optimizer/pathnode.h b/src/include/optimizer/pathnode.h index 99f65b44f22..3ef12b323bb 100644 --- a/src/include/optimizer/pathnode.h +++ b/src/include/optimizer/pathnode.h @@ -153,6 +153,7 @@ extern HashPath *create_hashjoin_path(PlannerInfo *root, JoinPathExtraData *extra, Path *outer_path, Path *inner_path, + bool parallel_hash, List *restrict_clauses, Relids required_outer, List *hashclauses); |