Re: Force specific index disuse

On Tue, May 20, 2014 at 11:48 AM, Steve Crawford <
scrawford(at)pinpointresearch(dot)com> wrote:

> On 05/20/2014 10:44 AM, Alvaro Herrera wrote:
>
>> Steve Crawford wrote:
>>
>>> Is there a way to force a specific index to be removed from
>>> consideration in planning a single query?
>>>
>>> Specifically, on a 60-million-row table I have an index that is a
>>> candidate for removal. I have identified the sets of nightly queries
>>> that use the index but before dropping it I would like to run
>>> EXPLAIN and do timing tests on the queries to see the impact of not
>>> having that index available and rewrite the query to efficiently use
>>> other indexes if necessary.
>>>
>> If you can afford to lock the table for a while, the easiest is
>>
>> BEGIN;
>> DROP INDEX bothersome_idx;
>> EXPLAIN your_query;
>> ROLLBACK;
>>
>> Interesting. But what do you mean by "a while?" Does the above keep the
> index intact (brief lock) or does it have to rebuild it on rollback?
>

Best case, 'A while' means however long it takes the explain (possibly
analyze) to run, and for you to then type 'rollback;'

worse case, someone else is already holding an incompatible lock (i.e. any
lock) on the table, and is going to hang on to it for a long while, so your
drop index hangs forever waiting to acquire the lock and in the process
brings all other desired activity (except the one already holding the lock)
to a screeching halt because they are not allowed to jump the lock queue.

worser case, you forget to enter 'rollback' at all and accidentally commit
the index drop.

I use the begin...drop...rollback on test servers a lot, but rarely on
productions servers. If I don't want to lock for that long, you can often
alter the query to make the index useless, for example:

explain select * from pgbench_accounts where aid=87;

Becomes:

explain select * from pgbench_accounts where aid+0=87;

Although for complex queries it can be hard to find the antimagic bullet.

Cheers,

Jeff