Re: slow join not using index properly

From: Vincent
Subject: Re: slow join not using index properly
Date: ,
Msg-id: 532BF5A3.7040802@xs4all.nl
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In response to: slow join not using index properly  (Stefan Amshey)
List: pgsql-performance

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slow join not using index properly  (Stefan Amshey, )
 Re: slow join not using index properly  (Ilya Kosmodemiansky, )
  Re: slow join not using index properly  (Stefan Amshey, )
   Re: slow join not using index properly  (Ilya Kosmodemiansky, )
 Re: slow join not using index properly  (Vincent, )

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?

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