Message-ID: <4DA87DAC.30006@jivesoftware.com>
Date: Fri, 15 Apr 2011 10:17:32 -0700
From: Mark Williams <mark.williams@jivesoftware.com>
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To: <pgsql-performance@postgresql.org>
Subject: Bad Query Plan with Range Query
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We are experiencing a problem with our query plans when using a range 
query in Postgresql 8.3. The query we are executing attempts to select 
the minimum primary key id after a certain date. Our date columns are 
bigint's holding a unix epoch representation of the date. We have an 
index on the primary key and the date column.

For the following query just specified the predicate modificationDate >= ?

explain SELECT min(messageID) FROM Message WHERE modificationDate >= 
1302627793988;
                                                QUERY PLAN
---------------------------------------------------------------------------------------------------------
  Result  (cost=2640.96..2640.97 rows=1 width=0)
    InitPlan
      ->  Limit  (cost=0.00..2640.96 rows=1 width=8)
            ->  Index Scan using message_pk on message  
(cost=0.00..3298561.09 rows=1249 width=8)
                  Filter: ((messageid IS NOT NULL) AND (modificationdate 
 >= 1302627793988::bigint))
(5 rows)

For some reason it is deciding to scan the primary key column of the 
table. This results in scanning the entire table which is huge (10 
million records).

However, if we specify a fake upper bound then the planner will 
correctly use the date column index:

explain SELECT min(messageID) FROM Message WHERE modificationDate >= 
1302627793988 and modificationDate < 9999999999999999;
                                                      QUERY PLAN
---------------------------------------------------------------------------------------------------------------------
  Aggregate  (cost=9.64..9.65 rows=1 width=8)
    ->  Index Scan using jvmssg_mdate_idx on message  (cost=0.00..9.64 
rows=1 width=8)
          Index Cond: ((modificationdate >= 1302627793988::bigint) AND 
(modificationdate < 9999999999999999::bigint))
(3 rows)

We have carried out all the usual maintenance tasks. We have increase 
the statistics_target on both indexes to the maximum (1000) and 
performed a vacuum analyze on the table. Our resource configurations are 
very good since this is our production server.

Interestingly this does not appear to happen with exactly the same 
database when using 8.4. Instead we get the correct plan without having 
to add the upper bound.

Here is the full description of the the table. It contains upwards of 10 
million rows.

               Table "public.message"
       Column      |          Type          | Modifiers
------------------+------------------------+-----------
  messageid        | bigint                 | not null
  parentmessageid  | bigint                 |
  threadid         | bigint                 | not null
  containertype    | integer                | not null
  containerid      | bigint                 | not null
  userid           | bigint                 |
  subject          | character varying(255) |
  body             | text                   |
  modvalue         | integer                | not null
  rewardpoints     | integer                | not null
  creationdate     | bigint                 | not null
  modificationdate | bigint                 | not null
  status           | integer                | not null
Indexes:
     "message_pk" PRIMARY KEY, btree (messageid)
     "jvmssg_cdate_idx" btree (creationdate)
     "jvmssg_cidctmd_idx" btree (containerid, containertype, 
modificationdate)
     "jvmssg_mdate_idx" btree (modificationdate)
     "jvmssg_mdvle_idx" btree (modvalue)
     "jvmssg_prntid_idx" btree (parentmessageid)
     "jvmssg_thrd_idx" btree (threadid)
     "jvmssg_usrid_idx" btree (userid)
Referenced by:
     TABLE "answer" CONSTRAINT "answer_mid_fk" FOREIGN KEY (messageid) 
REFERENCES message(messageid)
     TABLE "messageprop" CONSTRAINT "jmp_msgid_fk" FOREIGN KEY 
(messageid) REFERENCES message(messageid)


Any insight into this would be greatly appreciated. We are not able to 
upgrade our databases to 8.4. We are reluctant to re-write all our range 
queries if possible.


-m