Hi Vladimir,

On 03.09.2016 19:31, Vladimir Sitnikov wrote:
Ildar>The reason why this doesn't work is that '~~' operator (which is a
Ildar>synonym for 'like') isn't supported by operator class for btree. Since
Ildar>the only operators supported by btree are <, <=, =, >=, >, you can use
Ildar>it with queries like:

Ildar>And in 3rd query 'OFFSET' statement prevents rewriter from
Ildar>transforming the query, so it is possible to use index only scan on
Ildar>subquery and then filter the result of subquery with '~~' operator.

I'm afraid I do not follow you.
Note: query 3 is 100% equivalent of query 2, however query 3 takes 55
times less reads.
It looks like either an optimizer bug, or some missing feature in the
"index only scan" logic.

Here's quote from "query 2" (note % are at both ends):  ... where
type=42) as x where upper_vc like '%ABC%';

Note: I do NOT use "indexed scan" for the like operator. I'm very well aware
that LIKE patterns with leading % cannot be optimized to a btree range scan.
What I want is "use the first indexed column as index scan, then use the
second column
for filtering".

As shown in "query 2" vs "query 3", PostgreSQL cannot come up with such
a plan on its own
for some reason.

This is not a theoretical issue, but it is something that I use a lot
with Oracle DB (it just creates a good plan for "query 2").


Thanks, I get it now. The reason why it acts like this is that I used match_clause_to_index() function to determine if IOS can be used with the specified clauses. This function among other things checks if operator matches the index opfamily. Apparently this isn't correct. I wrote another prototype to test your case and it seems to work. But it's not ready for public yet, I'll publish it in 1-2 days.

Ildar Musin

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