Hello All
While investigating switching to Postgres, we come across a query plan
that uses hash join and is a lot slower than a nested loop join.
I don't understand why the optimiser chooses the hash join in favor of
the nested loop. What can I do to get the optimiser to make a better
decision (nested loop in this case)? I have run analyze on both tables.
Optimiser thinks that nested loop is more expensive, because of point PK lookups, which a random io. Can you set random_page_cost to 2 or 3 and try again?
The query is,
/*
smalltable has about 48,000 records.
bigtable has about 168,000,000 records.
invtranref is char(10) and is the primary key for both tables
*/
SELECT
*
FROM IM_Match_Table smalltable
inner join invtran bigtable on
bigtable.invtranref = smalltable.invtranref
The hash join plan is,
"Hash Join (cost=1681.87..6414169.04 rows=48261 width=171)"
" Output: smalltable.invtranref, smalltable.itbatchref,
smalltable.trantype, smalltable.trandate, smalltable.invprodref,
smalltable.invheadref, bigtable.itbatchref, bigtable.invtranref,
bigtable.invheadref, bigtable.feeplanref, bigtable.invprodref,
bigtable.trantype, bigtable.trandate, bigtable.pricedate,
bigtable.units, bigtable.tranamount, bigtable.createmode,
bigtable.transtat, bigtable.sysversion, bigtable.sysuser,
bigtable.rectype, bigtable.recstat, bigtable.seqnum, bigtable.transign"
" Hash Cond: (bigtable.invtranref = smalltable.invtranref)"
" -> Seq Scan on public.invtran bigtable (cost=0.00..4730787.28 rows=168121728 width=108)"
" Output: bigtable.itbatchref, bigtable.invtranref,
bigtable.invheadref, bigtable.feeplanref, bigtable.invprodref,
bigtable.trantype, bigtable.trandate, bigtable.pricedate,
bigtable.units, bigtable.tranamount, bigtable.createmode,
bigtable.transtat, bigtable.sysversion, bigtable.sysuser,
bigtable.rectype, bigtable.recstat, bigtable.seqnum, bigtable.transign"
" -> Hash (cost=1078.61..1078.61 rows=48261 width=63)"
" Output: smalltable.invtranref, smalltable.itbatchref,
smalltable.trantype, smalltable.trandate, smalltable.invprodref,
smalltable.invheadref"
" -> Seq Scan on public.im_match_table smalltable (cost=0.00..1078.61 rows=48261 width=63)"
" Output: smalltable.invtranref,
smalltable.itbatchref, smalltable.trantype, smalltable.trandate,
smalltable.invprodref, smalltable.invheadref"
The nested loop join plan is,
"Nested Loop (cost=0.00..12888684.07 rows=48261 width=171)"
" Output: smalltable.invtranref, smalltable.itbatchref,
smalltable.trantype, smalltable.trandate, smalltable.invprodref,
smalltable.invheadref, bigtable.itbatchref, bigtable.invtranref,
bigtable.invheadref, bigtable.feeplanref, bigtable.invprodref,
bigtable.trantype, bigtable.trandate, bigtable.pricedate,
bigtable.units, bigtable.tranamount, bigtable.createmode,
bigtable.transtat, bigtable.sysversion, bigtable.sysuser,
bigtable.rectype, bigtable.recstat, bigtable.seqnum, bigtable.transign"
" -> Seq Scan on public.im_match_table smalltable (cost=0.00..1078.61 rows=48261 width=63)"
" Output: smalltable.invtranref, smalltable.itbatchref,
smalltable.trantype, smalltable.trandate, smalltable.invprodref,
smalltable.invheadref"
" -> Index Scan using pk_invtran on public.invtran bigtable (cost=0.00..267.03 rows=1 width=108)"
" Output: bigtable.itbatchref, bigtable.invtranref,
bigtable.invheadref, bigtable.feeplanref, bigtable.invprodref,
bigtable.trantype, bigtable.trandate, bigtable.pricedate,
bigtable.units, bigtable.tranamount, bigtable.createmode,
bigtable.transtat, bigtable.sysversion, bigtable.sysuser,
bigtable.rectype, bigtable.recstat, bigtable.seqnum, bigtable.transign"
" Index Cond: (bigtable.invtranref = smalltable.invtranref)"
The version is PostgreSQL 9.2.0 on x86_64-unknown-linux-gnu, compiled by
gcc (GCC) 4.4.6 20120305 (Red Hat 4.4.6-4), 64-bit. Server specs are:
- Centos, ext4
- 24GB memory
- 6 cores hyper-threaded (Intel(R) Xeon(R) CPU E5645).
- raid 10 on 4 sata disks
Config changes are
- shared_buffers = 6GB
- effective_cache_size = 18GB
- work_mem = 10MB
- maintenance_work_mem = 3GB
Many Thanks
Huan
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