Re: View has different query plan than select statement

On Mon, May 19, 2014 at 4:47 PM, Geoff Hull <geoff(dot)hull(at)mccarthy(dot)co(dot)nz>wrote:

> I am sending this on behalf of my colleague who tried to post to this list
> last year but without success, then also tried
> pgsql-performance-owner(at)postgresql(dot)org but without getting a reply.
>
> I have recently re-tested this in P/G version 9.3.4 with the same results:
>
> Hi,
>
> I have created a table 'test_table' and index 'idx_test_table' with a view
> 'v_test_table'. However the query plan used by the view does not use the
> index but when running the select statement itself it does use the index.
> Given that query specific hints are not available in Postgres 9.1 how can I
> persuade the view to use the same query plan as the select statement?
>
> Thanks,
>
> Tim
>
>
> --DROP table test_table CASCADE;
>
> -- create test table
> CREATE TABLE test_table (
> history_id SERIAL,
> id character varying(50) NOT NULL ,
> name character varying(50),
> CONSTRAINT test_table_pkey PRIMARY KEY (history_id)
> );
>
> -- create index on test table
> CREATE INDEX idx_test_table ON test_table (id);
>
> -- populate test table
> INSERT INTO test_table (id, name) SELECT *, 'Danger Mouse' FROM (SELECT
> md5(random()::text) from generate_series(1,10000)) q;
>
> -- collect stats
> ANALYZE test_table;
>
>
> EXPLAIN (ANALYZE, BUFFERS)
> SELECT *
> FROM test_table
> WHERE id = '02b304b1c54542570d9f7bd39361f5b4';
>
> "Index Scan using idx_test_table on test_table (cost=0.00..8.27 rows=1
> width=50) (actual time=0.021..0.022 rows=1 loops=1)"
> " Index Cond: ((id)::text = '02b304b1c54542570d9f7bd39361f5b4'::text)"
> " Buffers: shared hit=3"
> "Total runtime: 0.051 ms"
>
>
> -- select statement with good plan
>
> EXPLAIN (ANALYZE, BUFFERS)
> SELECT id,
> CASE WHEN COALESCE(LAG(name) OVER (PARTITION BY id ORDER BY history_id),
> name || 'x') <> name
> then name
> end as name
> FROM test_table
> WHERE id = '02b304b1c54542570d9f7bd39361f5b4';
>
> "WindowAgg (cost=8.28..8.31 rows=1 width=50) (actual time=0.050..0.051
> rows=1 loops=1)"
> " Buffers: shared hit=3"
> " -> Sort (cost=8.28..8.29 rows=1 width=50) (actual time=0.039..0.039
> rows=1 loops=1)"
> " Sort Key: history_id"
> " Sort Method: quicksort Memory: 25kB"
> " Buffers: shared hit=3"
> " -> Index Scan using idx_test_table on test_table (cost=0.00..8.27 rows=1
> width=50) (actual time=0.030..0.031 rows=1 loops=1)"
> " Index Cond: ((id)::text = '02b304b1c54542570d9f7bd39361f5b4'::text)"
> " Buffers: shared hit=3"
> "Total runtime: 0.102 ms"
>
>
> --DROP VIEW v_test_table;
>
> CREATE OR REPLACE VIEW v_test_table AS
> SELECT id,
> CASE WHEN COALESCE(LAG(name) OVER (PARTITION BY id ORDER BY history_id),
> name || 'x') <> name
> then name
> end as name
> FROM test_table;
>
>
> -- Query via view with bad plan
>
> EXPLAIN (ANALYZE, BUFFERS)
> SELECT *
> FROM v_test_table
> WHERE id = '02b304b1c54542570d9f7bd39361f5b4';
>
> "Subquery Scan on v_test_table (cost=868.39..1243.39 rows=50 width=65)
> (actual time=26.115..33.327 rows=1 loops=1)"
> " Filter: ((v_test_table.id)::text =
> '02b304b1c54542570d9f7bd39361f5b4'::text)"
> " Buffers: shared hit=104, temp read=77 written=77"
> " -> WindowAgg (cost=868.39..1118.39 rows=10000 width=50) (actual time=
> 26.022..32.519 rows=10000 loops=1)"
> " Buffers: shared hit=104, temp read=77 written=77"
> " -> Sort (cost=868.39..893.39 rows=10000 width=50) (actual
> time=26.013..27.796 rows=10000 loops=1)"
> " Sort Key: test_table.id, test_table.history_id"
> " Sort Method: external merge Disk: 608kB"
> " Buffers: shared hit=104, temp read=77 written=77"
> " -> Seq Scan on test_table (cost=0.00..204.00 rows=10000 width=50)
> (actual time=0.010..1.804 rows=10000 loops=1)"
> " Buffers: shared hit=104"
> "Total runtime: 33.491 ms"
>
>
> How can I get the view to use the same query plan as the select statement?
>
>
Hi Geoff,

Unfortunately the view is not making use of the index due to the presence
of the windowing function in the view. I think you would find that if that
was removed then the view would more than likely use the index again.

The reason for this is that currently the WHERE clause of the outer query
is not pushed down into the view due to some overly strict code which
completely disallows pushdowns of where clauses into sub queries that
contain windowing functions...

In your case, because you have this id in your partition by clause, then
technically it is possible to push the where clause down into the sub
query. I wrote a patch a while back which lifts this restriction. it
unfortunately missed the boat for 9.4, but with any luck it will make it
into 9.5. If you're up for compiling postgres from source, then you can
test the patch out:

http://www.postgresql.org/message-id/CAHoyFK9ihoSarntWc-NJ5tPHko4Wcausd-1C_0wEcogi9UEKTw@mail.gmail.com

It should apply to current HEAD without too much trouble.

Regards

David Rowley