Thread: Lack of index usage when doing array casts
Hello,
We had a problem with PostgreSQL not using an index scan in 2 similar queries, the only difference between them is the array cast from text[] to location_type[] (array of enum values).
The execution plans are the following:
1.
Hash Join (cost=1.68..64194.88 rows=962149 width=62) (actual time=0.096..3580.542 rows=62 loops=1)
Hash Cond: (location.topology_id = topology.t_id)
-> Seq Scan on location (cost=0.00..34126.05 rows=962149 width=58) (actual time=0.031..3580.261 rows=62 loops=1)
Filter: (type = ANY (('{CITY,VILLAGE,TOWN,ROOM}'::text[])::location_type[]))
-> Hash (cost=1.30..1.30 rows=30 width=8) (actual time=0.041..0.041 rows=31 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 2kB
-> Seq Scan on topology (cost=0.00..1.30 rows=30 width=8) (actual time=0.005..0.019 rows=31 loops=1)
Total runtime: 3580.604 ms
2.
Hash Join (cost=29.91..3649.53 rows=1435 width=62) (actual time=0.366..0.811 rows=62 loops=1)
Hash Cond: (location.topology_id = topology.t_id)
-> Bitmap Heap Scan on location (cost=28.24..3603.01 rows=1435 width=58) (actual time=0.239..0.311 rows=62 loops=1)
Recheck Cond: (type = ANY ('{CITY,VILLAGE,TOWN,ROOM}'::location_type[]))
-> Bitmap Index Scan on location_type_idx (cost=0.00..27.88 rows=1435 width=0) (actual time=0.223..0.223 rows=62 loops=1)
Index Cond: (type = ANY ('{CITY,VILLAGE,TOWN,ROOM}'::location_type[]))
-> Hash (cost=1.30..1.30 rows=30 width=8) (actual time=0.076..0.076 rows=31 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 2kB
-> Seq Scan on topology (cost=0.00..1.30 rows=30 width=8) (actual time=0.019..0.041 rows=31 loops=1)
Total runtime: 0.934 ms
The problematic line is this one:
-> Seq Scan on location (cost=0.00..34126.05 rows=962149 width=58) (actual time=0.031..3580.261 rows=62 loops=1)
Filter: (type = ANY (('{CITY,VILLAGE,TOWN,ROOM}'::text[])::location_type[]))
The PostgreSQL version this query is running is 9.3.2.
Is it expected that index is not used during such a cast? If so, what would be the better way to force the index usage when doing array casts?
Sincerely,
-- Alexey Klyukin
Alexey Klyukin <alexk@hintbits.com> writes:
> We had a problem with PostgreSQL not using an index scan in 2 similar
> queries, the only difference between them is the array cast from text[] to
> location_type[] (array of enum values).
Hmm. IIRC the text to enum cast is considered stable not immutable, which
is why that doesn't get folded to a Const on sight. However, it seems
like it'd be okay for scalararraysel() to reduce stable expressions for
estimation purposes, ie it should be using estimate_expression_value.
regards, tom lane
I wrote:
> Hmm. IIRC the text to enum cast is considered stable not immutable, which
> is why that doesn't get folded to a Const on sight. However, it seems
> like it'd be okay for scalararraysel() to reduce stable expressions for
> estimation purposes, ie it should be using estimate_expression_value.
I've committed a patch for this; it will be in 9.3.4.
regards, tom lane
Thank you!
Hopefully I'll be able to give it a spin next week and will let you know whether the patch improved the execution plans in our environment.
Sincerely,
--
Alexey Klyukin
On Fri, Feb 21, 2014 at 2:37 PM, Alexey Klyukin <alexk@hintbits.com> wrote: > Hopefully I'll be able to give it a spin next week and will let you know > whether the patch improved the execution plans in our environment. 9.3.3 is out this week; you'll have to wait a few months for this if you're using standard packages, I'm afraid. -- Regards, Peter Geoghegan