mirror of https://github.com/citusdata/citus.git
4c3934272f
We were effectively joining on a calculated column because of our calls
to `shard_name`. This caused a really bad plan to be generated. In my
specific case it was taking ~18 seconds to show the output of
citus_shards. It had this explain plan:
```
QUERY PLAN
─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
Subquery Scan on citus_shards (cost=18369.74..18437.34 rows=5408 width=124) (actual time=18277.461..18278.509 rows=5408 loops=1)
-> Sort (cost=18369.74..18383.26 rows=5408 width=156) (actual time=18277.457..18277.726 rows=5408 loops=1)
Sort Key: ((pg_dist_shard.logicalrelid)::text), pg_dist_shard.shardid
Sort Method: quicksort Memory: 1629kB
CTE shard_sizes
-> Function Scan on citus_shard_sizes (cost=0.00..10.00 rows=1000 width=40) (actual time=71.137..71.934 rows=5413 loops=1)
-> Hash Join (cost=177.62..18024.42 rows=5408 width=156) (actual time=77.985..18257.237 rows=5408 loops=1)
Hash Cond: ((pg_dist_shard.logicalrelid)::oid = (pg_dist_partition.logicalrelid)::oid)
-> Hash Join (cost=169.81..371.98 rows=5408 width=48) (actual time=1.415..13.166 rows=5408 loops=1)
Hash Cond: (pg_dist_placement.groupid = pg_dist_node.groupid)
-> Hash Join (cost=168.68..296.49 rows=5408 width=16) (actual time=1.403..10.011 rows=5408 loops=1)
Hash Cond: (pg_dist_placement.shardid = pg_dist_shard.shardid)
-> Seq Scan on pg_dist_placement (cost=0.00..113.60 rows=5408 width=12) (actual time=0.004..3.684 rows=5408 loops=1)
Filter: (shardstate = 1)
-> Hash (cost=101.08..101.08 rows=5408 width=12) (actual time=1.385..1.386 rows=5408 loops=1)
Buckets: 8192 Batches: 1 Memory Usage: 318kB
-> Seq Scan on pg_dist_shard (cost=0.00..101.08 rows=5408 width=12) (actual time=0.003..0.688 rows=5408 loops=1)
-> Hash (cost=1.06..1.06 rows=6 width=40) (actual time=0.007..0.007 rows=6 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 9kB
-> Seq Scan on pg_dist_node (cost=0.00..1.06 rows=6 width=40) (actual time=0.004..0.005 rows=6 loops=1)
-> Hash (cost=5.69..5.69 rows=169 width=130) (actual time=0.070..0.071 rows=169 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 36kB
-> Seq Scan on pg_dist_partition (cost=0.00..5.69 rows=169 width=130) (actual time=0.009..0.041 rows=169 loops=1)
SubPlan 2
-> Limit (cost=0.00..3.25 rows=1 width=8) (actual time=3.370..3.370 rows=1 loops=5408)
-> CTE Scan on shard_sizes (cost=0.00..32.50 rows=10 width=8) (actual time=3.369..3.369 rows=1 loops=5408)
Filter: ((shard_name(pg_dist_shard.logicalrelid, pg_dist_shard.shardid) = table_name) OR (('public.'::text || shard_name(pg_dist_shard.logicalrelid, pg_dist_shard.shardid)) = table_name))
Rows Removed by Filter: 2707
Planning Time: 0.705 ms
Execution Time: 18278.877 ms
```
With the changes it only takes 180ms to show the same output:
```
QUERY PLAN
──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
Sort (cost=904.59..918.11 rows=5408 width=156) (actual time=182.508..182.960 rows=5408 loops=1)
Sort Key: ((pg_dist_shard.logicalrelid)::text), pg_dist_shard.shardid
Sort Method: quicksort Memory: 1629kB
-> Hash Join (cost=418.03..569.27 rows=5408 width=156) (actual time=136.333..146.591 rows=5408 loops=1)
Hash Cond: ((pg_dist_shard.logicalrelid)::oid = (pg_dist_partition.logicalrelid)::oid)
-> Hash Join (cost=410.22..492.83 rows=5408 width=56) (actual time=136.231..140.132 rows=5408 loops=1)
Hash Cond: (pg_dist_placement.groupid = pg_dist_node.groupid)
-> Hash Right Join (cost=409.09..417.34 rows=5408 width=24) (actual time=136.218..138.890 rows=5408 loops=1)
Hash Cond: ((((regexp_matches(citus_shard_sizes.table_name, '_(\d+)$'::text))[1])::integer) = pg_dist_shard.shardid)
-> HashAggregate (cost=45.00..48.50 rows=200 width=12) (actual time=131.609..132.481 rows=5408 loops=1)
Group Key: ((regexp_matches(citus_shard_sizes.table_name, '_(\d+)$'::text))[1])::integer
Batches: 1 Memory Usage: 737kB
-> Result (cost=0.00..40.00 rows=1000 width=12) (actual time=107.786..129.831 rows=5408 loops=1)
-> ProjectSet (cost=0.00..22.50 rows=1000 width=40) (actual time=107.780..128.492 rows=5408 loops=1)
-> Function Scan on citus_shard_sizes (cost=0.00..10.00 rows=1000 width=40) (actual time=107.746..108.107 rows=5414 loops=1)
-> Hash (cost=296.49..296.49 rows=5408 width=16) (actual time=4.595..4.598 rows=5408 loops=1)
Buckets: 8192 Batches: 1 Memory Usage: 339kB
-> Hash Join (cost=168.68..296.49 rows=5408 width=16) (actual time=1.702..3.783 rows=5408 loops=1)
Hash Cond: (pg_dist_placement.shardid = pg_dist_shard.shardid)
-> Seq Scan on pg_dist_placement (cost=0.00..113.60 rows=5408 width=12) (actual time=0.004..0.837 rows=5408 loops=1)
Filter: (shardstate = 1)
-> Hash (cost=101.08..101.08 rows=5408 width=12) (actual time=1.683..1.685 rows=5408 loops=1)
Buckets: 8192 Batches: 1 Memory Usage: 318kB
-> Seq Scan on pg_dist_shard (cost=0.00..101.08 rows=5408 width=12) (actual time=0.004..0.824 rows=5408 loops=1)
-> Hash (cost=1.06..1.06 rows=6 width=40) (actual time=0.007..0.008 rows=6 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 9kB
-> Seq Scan on pg_dist_node (cost=0.00..1.06 rows=6 width=40) (actual time=0.004..0.006 rows=6 loops=1)
-> Hash (cost=5.69..5.69 rows=169 width=130) (actual time=0.079..0.079 rows=169 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 36kB
-> Seq Scan on pg_dist_partition (cost=0.00..5.69 rows=169 width=130) (actual time=0.011..0.046 rows=169 loops=1)
Planning Time: 0.789 ms
Execution Time: 184.095 ms
```
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| backend | ||
| include | ||
| test | ||