Re: BUG #17922: ANY vs IN execution plan difference for a single explicit input value

PG Bug reporting form <noreply@postgresql.org> writes:
> I guess there can be differences how IN vs ANY input is handled for prepared
> statements but for plain SQL input I would expect them to produce identical
> plans.

If you can find any such promise in the documentation, I will be glad
to remove it ;-)

> EXPLAIN
> SELECT
>     a.*
> FROM
>     a
>     JOIN b USING (node_id)
> WHERE
>     a.project_id = ANY (ARRAY[1]::int8[])
> ORDER BY
>     a.project_id,
>     a.node_id
> LIMIT 10;

> EXPLAIN
> SELECT
>     a.*
> FROM
>     a
>     JOIN b USING (node_id)
> WHERE
>     a.project_id IN (1) -- Also OK with a plain "=1" witout IN
> ORDER BY
>     a.project_id,
>     a.node_id
> LIMIT 10;

The thing about "project_id IN (1)", which is reduced to "project_id = 1"
by the parser, is that the planner can see that that renders the "ORDER
BY a.project_id" clause a no-op.  So it only has to order by node_id,
which is what makes this plan valid:

>  Limit  (cost=0.72..38.07 rows=10 width=16)
>    ->  Nested Loop  (cost=0.72..55440.79 rows=14842 width=16)
>          ->  Index Only Scan using b_node_id_key on b  (cost=0.29..397.29 rows=15000 width=8)
>          ->  Index Only Scan using a_uq on a  (cost=0.43..3.67 rows=1 width=16)
>                Index Cond: ((project_id = 1) AND (node_id = b.node_id))

If you don't recognize that then you're forced into sorting the indexscan
output, which is going to look pretty bad for a small-LIMIT situation.

We don't make any attempt to make a similar deduction from =ANY clauses,
mainly because it's usually not possible to be sure that the array has
only one member.  I'm not excited about the cost/benefit ratio of adding
code to check for that.

            regards, tom lane