Re: Multicolumn index for single-column queries?

On 19/04/2019 01:47, Harald Fuchs wrote:
> Andreas Kretschmer<andreas(at)a-kretschmer(dot)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.
>
The number of rows referenced by an index entry for the multi_index will
always be less than or equal to those for the matching foo_index.

Also there will be fewer index entries per block for the multi_index,
which is why the I/O count will be higher even in the best case where
there is an equal number of rows referenced by each index entry.