Re: Multicolumn index for single-column queries?

Andreas Kretschmer <andreas@a-kretschmer.de> writes:

> Am 18.04.19 um 08:52 schrieb rihad:
>> Hi. Say there are 2 indexes:
>>
>>     "foo_index" btree (foo_id)
>>
>>     "multi_index" btree (foo_id, approved, expires_at)
>>
>>
>> foo_id is an integer. Some queries involve all three columns in
>> their WHERE clauses, some involve only foo_id.
>> Would it be ok from general performance standpoint to remove
>> foo_index and rely only on multi_index? I know that
>> PG would have to do less work updating just one index compared to
>> updating them both, but wouldn't searches
>> on foo_id alone become slower? 
>
> it depends .
>
> it depends on the queries you are using, on your workload. a
> multi-column-index will be large than an index over just one column,
> therefore you will have more disk-io when you read from such an index.

I think it also depends on the average number of rows having the same foo_id.