Re: Question about DROP TABLE

Hi Michal,

This isn't really a question for -hackers, the list for postgres
development. -general or -performance would have been more appropriate.

On 2016-01-12 11:57:05 +0100, Michal Novotny wrote:
> I've discovered an issue with dropping a large table (~5T). I was
> thinking drop table is fast operation however I found out my assumption
> was wrong.

What exactly did you do, and how long did it take. Is there any chance
you were actually waiting for the lock on that large table, instead of
waiting for the actual execution?

> Is there any way how to tune it to drop a large table in the matter of
> seconds or minutes? Any configuration variable in the postgresql.conf or
> any tune up options available?

The time for dropping a table primarily is spent on three things:
1) acquiring the exclusive lock. How long this takes entirely depends on
the concurrent activity. If there's a longrunning session using that
table it'll take till that session is finished.
2) The cached portion of that table needs to be eviced from cache. How
long that takes depends on the size of shared_buffers - but usually
this is a relatively short amount of time, and only matters if you
drop many, many relations.
3) The time the filesystem takes to actually remove the, in your case
5000 1GB, files. This will take a while, but shouldn't be minutes.

Greetings,

Andres Freund