Re: Odd optimiser behaviour

Christopher Kings-Lynne wrote:
> EXPLAIN
> usa=# explain analyze UPDATE users_users SET suspended=false,
> suspended_on=NULL, suspended_off=NULL WHERE suspended_off=CURRENT_DATE;
> NOTICE: QUERY PLAN:
>
> Seq Scan on users_users (cost=0.00..2927.26 rows=267 width=248) (actual
> time=466.76..466.76 rows=0 loops=1)
> Total runtime: 467.02 msec
>
> EXPLAIN
>
> And now I'm always getting sequential scans. What gives? I analyze the
> table between runs.
>

In gram.y I see that CURRENT_DATE is transformed to 'now'::text::date. Here's
the comment:

* We cannot use "'now'::date" because coerce_type() will
* immediately reduce that to a constant representing
* today's date. We need to delay the conversion until
* runtime, else the wrong things will happen when
* CURRENT_DATE is used in a column default value or rule.

So I'm guessing that the optimizer sees this as volatile and therefore not
something it can use an index for. Try using now()::date instead, or maybe
wrap the call to CURRENT_DATE in a function of your own and mark it stable.

Joe