Why is RELEASE SAVEPOINT sometimes slow?

[Evan Martin <postgresql@xxxxxxxxxxxxxxxxx>]

I'm running a bulk import application against PostgreSQL 9.1.3, which 
has several stages and each stage follows the same general pattern:

BEGIN TRANSACTION
DELETE (many rows)

CREATE SAVEPOINT
INSERT
...
RELEASE SAVEPOINT

CREATE SAVEPOINT
INSERT
INSERT
... half an hour of inserts later ...
RELEASE SAVEPOINT

COMMIT TRANSACTION

I find that for one particular stage of the import the RELEASE SAVEPOINT 
command consistently takes about 6 minutes, while for the rest of them 
it's very quick. COMMIT TRANSACTION is always very quick.

At first I thought the discrepancy may be because that particular stage 
has many INSERT statements inside one savepoint, while other stages 
create many savepoints with a small amount of work in each. However, if 
I take out the savepoints entirely then the COMMIT TRANSACTION statement 
for that stage takes 6 minutes, while for the rest of them it's still 
very quick.

Could anyone explain what may be happening here? What is PostgreSQL 
doing when I call RELEASE SAVEPOINT that it seems to otherwise do in 
COMMIT TRANSACTION?

Thanks,

Evan

--
Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general