Thread: plpgsql: penalty due to double evaluation of parameters

Hi, <br /><br />Within exec_prepare_plan there are calls to exec_eval_datum to identify the argtypes of the involved
parameters.However exec_eval_datum actually fills up value, isnull entries in these cases causing unnecessary
additionalcalls when all we need is the datum type. Such unnecessary evaluation of values might prove to be very costly
latersince this quirk of exec_eval_datum usage is not so visible. Worse still it could cause bugs if some evaluations
haveside-effects across multiple evals. It might make sense to introduce a new function exec_eval_datum_type to address
thisor exec_eval_datum could itself be modified for cases where we just need the datum type. Should I cook up a patch
forthis? I am inclined towards introducing a new function (but that means that any new datum related changes need to be
carriedout in 2 functions instead of one currently).<br clear="all" /><br />Regards,<br />Nikhils<br />-- <br
/>EnterpriseDB              <a href="http://www.enterprisedb.com">http://www.enterprisedb.com</a> 

Nikhils wrote:
> Within exec_prepare_plan there are calls to exec_eval_datum to identify the
> argtypes of the involved parameters. However exec_eval_datum actually fills
> up value, isnull entries in these cases causing unnecessary additional calls
> when all we need is the datum type. Such unnecessary evaluation of values
> might prove to be very costly later since this quirk of exec_eval_datum
> usage is not so visible. Worse still it could cause bugs if some evaluations
> have side-effects across multiple evals.

I don't buy the performance argument unless I see some test results 
demonstrating it; exec_prepare_plan is only called on the first 
invocation of a statement. What kind of side-effects could 
exec_eval_datum call have?

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com

Hi,

Note that I have avoided using the "performance" word for this very reason. But consider for example when the datum type is PLPGSQL_DTYPE_REC. I dont think its justified to have the overhead of heap_copytuple_with_tuple, when all we need is just the typeid! Similar arguments apply for other datums like PLPGSQL_DTYPE_ROW, PLPGSQL_DTYPE_TRIGARG e.g.

Regards,
Nikhils
--
EnterpriseDB http://www.enterprisedb.com

"Heikki Linnakangas" <heikki@enterprisedb.com> writes:
> I don't buy the performance argument unless I see some test results 
> demonstrating it; exec_prepare_plan is only called on the first 
> invocation of a statement. What kind of side-effects could 
> exec_eval_datum call have?

In principle, if you subscript TG_ARGV[] with an expression that has
visible side-effects, you could have unexpected behavior here.  I think
though that the right fix is to get rid of the special treatment of
TG_ARGV[] and make it a ordinary variable with type text[], instead
of being its very own class of datum.  The current implementation has
a lot of other misbehaviors for TG_ARGV[], like not being able to apply
array operations to it.

I agree that worrying about the performance is pointless, considering
the cost of the SPI_prepare_cursor() that we're about to do in the one
place where there could be a use for a get-the-type-only call.
        regards, tom lane