Question

Postgres -- terrible performance with prepared statement

Asked by: JerryNorton

I have a performance problem with a prepared statement. Executing the prepared statement runs 800 ms(consistently). The sql statement takes less than one ms.  It appears that the prepared statement does a sequential scan of table, even though it should use index. Table is currently 1.2 million rows.

TABLE: INFO:  "users": scanned 3000 of 32866 pages, containing 111000 live rows and 0 dead rows; 3000 rows in sample, 1216042 estimated total rows

#PREPARE getUIDFromPasscode(char(20)) AS SELECT userid FROM users WHERE passcode=$1;
PREPARE
#explain EXECUTE getUIDFromPasscode('20000112');
                       QUERY PLAN
---------------------------------------------------------
 Seq Scan on users  (cost=0.00..48066.50 rows=1 width=8)
   Filter: ((passcode)::bpchar = $1)
(2 rows)

# explain SELECT userid FROM users WHERE passcode='20000112';
                                  QUERY PLAN
-------------------------------------------------------------------------------
 Index Scan using uq_users_passcode on users  (cost=0.00..3.01 rows=1 width=8)
   Index Cond: ((passcode)::text = '20000112'::text)
(2 rows)

vappscb=# explain EXECUTE getUIDFromPasscode(20000112);
                       QUERY PLAN
---------------------------------------------------------
 Seq Scan on users  (cost=0.00..48066.50 rows=1 width=8)
   Filter: ((passcode)::bpchar = $1)
(2 rows)

vappscb=#
vappscb=# explain EXECUTE getUIDFromPasscode('20000112'::text);
                       QUERY PLAN
---------------------------------------------------------
 Seq Scan on users  (cost=0.00..48066.50 rows=1 width=8)
   Filter: ((passcode)::bpchar = $1)
(2 rows)
 

as you can see, the cost of the sql is 3 and the cost of the prepared query is 48066.   I hope to get explanation and cure....

Jerry

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Asked On
2009-10-07 at 06:10:08ID24792157
Tags

postgres

,

db

,

postgresql

,

prepared statement

Topic

PostgreSQL Database

Participating Experts
1
Points
0
Comments
5

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Answers

 

by: earthman2Posted on 2009-10-07 at 08:15:43ID: 25516427

vappscb=# vacuum full analyze;

 

by: JerryNortonPosted on 2009-10-07 at 08:24:20ID: 25516541

Earthman, that's already answered in the post:

INFO:  "users": scanned 3000 of 32866 pages, containing 111000 live rows and 0 dead rows; 3000 rows in sample, 1216042 estimated total rows
no dead rows...stats are fresh.

Actually someone here found the answer:

Postgres' type hinting is the culprit. Passcode is a varchar(20) typed field. Preparing getUIDFromPasscode as given results in a prepared query that takes a char(20) which is then internally/implicitly converted back into an integer during the binding, and then converted back into a varchar(20) for actual lookup/comparison purposes.

As evidence supporting this, I offer the following, performed on asgqa3's database:

**** NOTE HERE the '' marks around $1 **** vappscb=# PREPARE getUIDFromPasscode(char(20)) AS SELECT userid FROM users WHERE passcode='$1'; PREPARE vappscb=# explain EXECUTE getUIDFromPasscode('20000112');
                                  QUERY PLAN
-------------------------------------------------------------------------------
 Index Scan using uq_users_passcode on users  (cost=0.00..3.01 rows=1 width=8)
   Index Cond: ((passcode)::text = '$1'::text)
(2 rows)

vappscb=# explain SELECT userid FROM users WHERE passcode='20000112';
                                  QUERY PLAN
-------------------------------------------------------------------------------
 Index Scan using uq_users_passcode on users  (cost=0.00..3.01 rows=1 width=8)
   Index Cond: ((passcode)::text = '20000112'::text)
(2 rows)

 

by: JerryNortonPosted on 2009-10-07 at 08:26:02ID: 25516550

seems good to me.

 

by: earthman2Posted on 2009-10-07 at 08:33:52ID: 25516631

The documention has a big  get out statement !
In some situations, the query plan produced for a prepared statement will be inferior to the query plan that would have been chosen if the statement had been submitted and executed normally. This is because when the statement is planned and the planner attempts to determine the optimal query plan, the actual values of any parameters specified in the statement are unavailable. PostgreSQL collects statistics on the distribution of data in the table, and can use constant values in a statement to make guesses about the likely result of executing the statement. Since this data is unavailable when planning prepared statements with parameters, the chosen plan might be suboptimal.

Check to see if index is marked as invalid.
vappscb=# \d users;
Check that column passcode has constraint not null.

If all else fails just use a simple select !  Prepared statements are really good for complex join logic.

 

by: earthman2Posted on 2009-10-07 at 08:48:41ID: 25516771

Do you get the same behaviour if you use text instead of char(20) in prepare statement argument ?
ie
PREPARE getUIDFromPasscode(text) AS SELECT userid FROM users WHERE passcode=$1;

20120131-EE-VQP-002

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