On 21-03-14 00:56, Stefan Amshey wrote:
> We have a slow performing query that we are trying to improve, and it
> appears to be performing a sequential scan at a point where it should
> be utilizing an index. Can anyone tell me why postgres is opting to do
> it this way?
>
> The original query is as follows:
>
> SELECT DISTINCT
> a1.context_key
> FROM
> virtual_ancestors a1, collection_data, virtual_ancestors a2
> WHERE
> a1.ancestor_key = collection_data.context_key
> AND collection_data.collection_context_key = a2.context_key
> AND a2.ancestor_key = ?
>
> The key relationships should all using indexed columns, but the query
> plan that postgres comes up with ends up performing a sequential scan
> on the collection_data table (in this case about 602k rows) where we
> would have expected it to utilize the index:
>
> HashAggregate (cost=60905.73..60935.73 rows=3000 width=4) (actual
> time=3366.165..3367.354 rows=3492 loops=1)
> Buffers: shared hit=16291 read=1222
> -> Nested Loop (cost=17546.26..60898.23 rows=3000 width=4) (actual
> time=438.332..3357.918 rows=13037 loops=1)
> Buffers: shared hit=16291 read=1222
> -> Hash Join (cost=17546.26..25100.94 rows=98 width=4) (actual
> time=408.554..415.767 rows=2092 loops=1)
> Hash Cond: (a2.context_key =
> collection_data.collection_context_key)
> Buffers: shared hit=4850 read=3
> -> Index Only Scan using virtual_ancestors_pkey on
> virtual_ancestors a2 (cost=0.00..233.32 rows=270 width=4) (actual
> time=8.532..10.703 rows=1960 loops=1)
> Index Cond: (ancestor_key = 1072173)
> Heap Fetches: 896
> Buffers: shared hit=859 read=3
> -> Hash (cost=10015.56..10015.56 rows=602456 width=8)
> (actual time=399.708..399.708 rows=602570 loops=1)
> Buckets: 65536 Batches: 1 Memory Usage: 23538kB
> Buffers: shared hit=3991
> ######## sequential scan occurs here ##########
> -> Seq Scan on collection_data (cost=0.00..10015.56 rows=602456
> width=8) (actual time=0.013..163.509 rows=602570 loops=1)
> Buffers: shared hit=3991
> -> Index Only Scan using virtual_ancestors_pkey on
> virtual_ancestors a1 (cost=0.00..360.70 rows=458 width=8) (actual
> time=1.339..1.403 rows=6 loops=2092)
> Index Cond: (ancestor_key = collection_data.context_key)
> Heap Fetches: 7067
> Buffers: shared hit=11441 read=1219
> Total runtime: 3373.058 ms
>
>
> The table definitions are as follows:
>
> Table "public.virtual_ancestors"
> Column | Type | Modifiers
> --------------+----------+-----------
> ancestor_key | integer | not null
> context_key | integer | not null
> degree | smallint | not null
> Indexes:
> "virtual_ancestors_pkey" PRIMARY KEY, btree (ancestor_key,
> context_key)
> "virtual_context_key_idx" btree (context_key)
> Foreign-key constraints:
> "virtual_ancestors_ancestor_key_fkey" FOREIGN KEY (ancestor_key)
> REFERENCES contexts(context_key)
> "virtual_ancestors_context_key_fkey" FOREIGN KEY (context_key)
> REFERENCES contexts(context_key)
>
> Table "public.collection_data"
> Column | Type | Modifiers
> ------------------------+----------------------+-----------
> collection_context_key | integer | not null
> context_key | integer | not null
> type | character varying(1) | not null
> source | character varying(1) | not null
> Indexes:
> "collection_data_context_key_idx" btree (context_key)
> "collection_data_context_key_index" btree (collection_context_key)
> CLUSTER
> Foreign-key constraints:
> "collection_data_collection_context_key_fkey" FOREIGN KEY
> (collection_context_key) REFERENCES contexts(context_key) ON DELETE
> CASCADE
> "collection_data_context_key_fkey" FOREIGN KEY (context_key)
> REFERENCES contexts(context_key) ON DELETE CASCADE
>
> Can anyone suggest a way that we can get postgres to use the
> collection_data_context_key_index properly? I thought that it might be
> related to the fact that collection_data_context_key_index is a
> CLUSTERED index, but we did some basic experimentation that seems to
> indicate otherwise, i.e. the bad plan persists despite re-clustering
> the index.
>
> We are using PostgreSQL 9.2.5 on x86_64-unknown-linux-gnu, compiled by
> gcc (Debian 4.4.5-8) 4.4.5, 64-bit
>
> Interestingly, on an instance running PostgreSQL 9.2.4 on
> x86_64-unknown-linux-gnu, compiled by gcc (Debian 4.4.5-8) 4.4.5,
> 64-bit where I copied the 2 tables over to a temporary database, the
> plan comes out differently:
>
> HashAggregate (cost=39692.03..39739.98 rows=4795 width=4) (actual
> time=73.285..75.141 rows=3486 loops=1)
> Buffers: shared hit=22458
> -> Nested Loop (cost=0.00..39680.05 rows=4795 width=4) (actual
> time=0.077..63.116 rows=13007 loops=1)
> Buffers: shared hit=22458
> -> Nested Loop (cost=0.00..32823.38 rows=164 width=4)
> (actual time=0.056..17.685 rows=2084 loops=1)
> Buffers: shared hit=7529
> -> Index Only Scan using virtual_ancestors_pkey on
> virtual_ancestors a2 (cost=0.00..1220.85 rows=396 width=4) (actual
> time=0.025..2.732 rows=1954 loops=1)
> Index Cond: (ancestor_key = 1072173)
> Heap Fetches: 1954
> Buffers: shared hit=1397
> ######## Note the index scan here - this is what it SHOULD be doing
> ##############
> -> Index Scan using collection_data_context_key_index on
> collection_data (cost=0.00..79.24 rows=56 width=8) (actual
> time=0.004..0.005 rows=1 loops=1954)
> Index Cond: (collection_context_key = a2.context_key)
> Buffers: shared hit=6132
> -> Index Only Scan using virtual_ancestors_pkey on
> virtual_ancestors a1 (cost=0.00..35.40 rows=641 width=8) (actual
> time=0.007..0.015 rows=6 loops=2084)
> Index Cond: (ancestor_key = collection_data.context_key)
> Heap Fetches: 13007
> Buffers: shared hit=14929
> Total runtime: 76.431 ms
>
> Why can't I get the Postgres 9.2.5 instance to use the optimal plan?
>
> Thanks in advance!
> /Stefan
>
> --
> -
> Stefan Amshey
The first plan expects to process 600000 rows in the sequential scan,
and the second plan expects to process only one,
so it looks like the statistics in the first database are out of date,
did you run vacuum analyse?