Re: Unused index influencing sequential scan plan

Thom Brown <thom@linux.com> writes:
> On 18 October 2012 17:44, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Thom Brown <thom@linux.com> writes:
>>> And as a side note, how come it's impossible to get the planner to use
>>> an index-only scan to satisfy the query (disabling sequential and
>>> regular index scans)?

>> Implementation restriction - we don't yet have a way to match index-only
>> scans to expressions.

> Ah, I suspected it might be, but couldn't find notes on what scenarios
> it's yet to be able to work in.  Thanks.

I forgot to mention that there is a klugy workaround: add the required
variable(s) as extra index columns.  That is,

    create index i on t (foo(x), x);

The planner isn't terribly bright about this, but it will use that index
for a query that only requires foo(x), and it won't re-evaluate foo()
(though I think it will cost the plan on the assumption it does :-().

            regards, tom lane