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Re: Select max(id) causes AccessExclusiveLock?

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Em 28/04/2014 12:01, Tom Lane escreveu:
Edson Richter <edsonrichter@xxxxxxxxxxx> writes:
When I issue a
select max(id) from MyTableName
Postgres 9.2.7 is scaling a AccessExclusiveLock and causing large delays.
Really?

regression=# create table MyTableName (
regression(#    id integer not null primary key,
regression(#    seqYear char(4),
regression(#    seqCount integer);
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "mytablename_pkey" for table "mytablename"
CREATE TABLE
regression=# begin;
BEGIN
regression=# select max(id) from MyTableName;
  max
-----
(1 row)

regression=# select * from pg_locks;
   locktype  | database | relation | page | tuple | virtualxid | transactionid | classid | objid | objsubid | virtualtransaction |  pid  |      mode       | granted | fastpath
------------+----------+----------+------+-------+------------+---------------+---------+-------+----------+--------------------+-------+-----------------+---------+----------
  relation   |    16384 |    11069 |      |       |            |               |         |       |          | 2/120853           | 12432 | AccessShareLock | t       | t
  relation   |    16384 |    42142 |      |       |            |               |         |       |          | 2/120853           | 12432 | AccessShareLock | t       | t
  relation   |    16384 |    42139 |      |       |            |               |         |       |          | 2/120853           | 12432 | AccessShareLock | t       | t
  virtualxid |          |          |      |       | 2/120853   |               |         |       |          | 2/120853           | 12432 | ExclusiveLock   | t       | t
(4 rows)

I see nothing higher than AccessShareLock on the table.

			regards, tom lane



Really big sorry!!!

The programmer added a


LOCK TABLE MyTableName


just before issuing the select max(id) from MyTableName.


I do suspect this is the case, right?

Really sorry, I should have look into the code before asking!

Thanks for your (always) fast support.

Regards,


Edson



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