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?