Re: Index scan cost calculation

> From: Jim Nasby 
>To: Jeff Janes ; Glyn Astill  
>Cc: Pgsql-performance 
>Sent: Wednesday, 2 December 2015, 22:32
>Subject: Re: [PERFORM] Index scan cost calculation
> 
>
>On 11/30/15 5:03 PM, Jeff Janes wrote:
>> It thinks the combination of (show, type, best, block)  is enough to
>> get down to a single row.  One index adds "flag" to that (which is not
>> useful to the query) and the other adds "row" to that, which is useful
>> but the planner doesn't think it is because once you are down to a
>> single tuple additional selectivity doesn't help.
>
>It occurs to me that maybe you could force this behavior by building an 
>index on a row() instead of on the individual fields. IE:
>
>CREATE INDEX ... ON( row(show, type, best, block, row) )
>
>You would then have to query based on that:
>
>WHERE row(show, type, best, block, row) = row( 'Trans Siberian 
>Orchestra', 'Music', true, 1, 1 )
>
>You mentioned legacy code which presumably you can't modify to do that, 
>but maybe there's a way to trick the planner into it with a view...
>
>CREATE VIEW AS
>SELECT r.show, r.type, r..., etc, etc
>   FROM ( SELECT *, row(show, type, best, block, row) AS r FROM table ) a
>;
>
>When you stick a where clause on that there's a chance it'd get turned 
>into WHERE row() = row()... but now that I see it I'm probably being 
>over optimistic about that. You could probably force the issue with an 
>ON SELECT ON table DO INSTEAD rule, but IIRC those aren't supported.


Thanks, interesting idea, but no cigar.

For the moment just ensuring the seats_index01 is the last index created seems to suffice, fragile though it is.