On Tue, May 20, 2014 at 11:48 AM, Steve Crawford < scrawf...@pinpointresearch.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
