so 8. 2. 2020 v 17:49 odesílatel Tom Lane <tgl@sss.pgh.pa.us> napsal:
Pavel Stehule <pavel.stehule@gmail.com> writes: > so 8. 2. 2020 v 7:44 odesílatel PG Bug reporting form < > noreply@postgresql.org> napsal: >> With '::text' type casting of '2020-02-08' (which is already text) query >> permofance is very low
> The '2020-02-08' is not text type - it is 'unknown' type - and then is just > directly transformed to date. I think so cast from text to date is not > maybe immutable, and it can stops some optimizations.
Yeah. There actually isn't any cast from text to date, if you look into pg_cast. So "('2020-02-08'::text)::date" is implemented as a text Const that's fed through a CoerceViaIO node that applies date_in(), and date_in() is only stable not immutable. (That must be so because its behavior depends on the DateStyle setting, and maybe TimeZone too; not sure about the latter but definitely the former.) So the planner is unable to reduce the IS NULL test to constant-false and thereby get rid of the OR, and that means it can't usefully apply the index.
If you can't rearrange things so that the IS NULL argument is seen as a constant, the UNION trick that Pavel mentioned might be a useful workaround. But I'm inclined to think that you need to take two steps back and figure out whether this query logic is really sane or not. You do realize that the query is asking to retrieve the entire table, if whatever-it-is is NULL? Why would that be what you want?
If I remember well, this technique was a trick to use one query for variables that can be (or should not be) specified by user.
I can has a variable $ID. If user specifies this variable, it has some number, else it has NULL.
When you want to use one query for both possibilities (static query), then you can write
SELECT * FROM tab WHERE ($ID is NULL OR id = $ID)
We used this technique 20 years ago, and I think it was very popular, but databases was significantly smaller, and only few people had good knowledge of SQL databases.
