On 20/04/14 12:08, Thomas Kellerer wrote:
>> Example:
>>
>> btree (fmb_id, action_type)
>> btree (fmb_id)
>>
[...]
>
> Btw: in the above example the second one is definitely not needed.
> Any query that uses the second one (single column) can also use the
> first one.
I know.
But the single column index is 3534 MB the 2-column one 4963 MB. The
number of rows per distinct fmb_id is very small, usually 1 or 2. So, if
a query looks for "fmb_id=A and action_type=B", it has to filter out
only one row if the first index is not available. Hence, I thought maybe
the lower size of the index and the higher compactness per page could
outweigh the more direct access provided by the 2-column index.
I am quite sure there is no query that qualifies for an index-only scan
on the 2-column index.
Torsten
