15.06.2023 16:58, c...@anastigmatix.net пишет:
On 2023-06-15 09:21, Tom Lane wrote:
Yura Sokolov <y.soko...@postgrespro.ru> writes:
not enough to be sure function doesn't manipulate data.
Of course not. It is the user's responsibility to mark functions
properly.
And also, isn't it the case that IMMUTABLE should mark a function,
not merely that "doesn't manipulate data", but whose return value
doesn't depend in any way on data (outside its own arguments)?
The practice among PLs of choosing an SPI readonly flag based on
the IMMUTABLE/STABLE/VOLATILE declaration seems to be a sort of
peculiar heuristic, not something inherent in what that declaration
means to the optimizer. (And also influences what snapshot the
function is looking at, and therefore what it can see, which has
also struck me more as a tacked-on effect than something inherent
in the declaration's meaning.)
Documentation disagrees:
https://www.postgresql.org/docs/current/sql-createfunction.html#:~:text=IMMUTABLE%0ASTABLE%0AVOLATILE
> |IMMUTABLE|indicates that the function cannot modify the database and
always returns the same result when given the same argument values
> |STABLE|indicates that the function cannot modify the database, and
that within a single table scan it will consistently return the same
result for the same argument values, but that its result could change
across SQL statements.
> |VOLATILE|indicates that the function value can change even within a
single table scan, so no optimizations can be made... But note that any
function that has side-effects must be classified volatile, even if its
result is quite predictable, to prevent calls from being optimized away
