Re: Optimisation deficiency: currval('seq')-->seq scan, constant-->index scan

At 02:18 PM 8/20/00 -0400, Tom Lane wrote:

>However, that just shows that some patterns of usage of the function
>will yield unpredictable results.  I don't think that translates to an
>argument that the optimizer is allowed to make semantics-altering
>transformations...

Very much depends on the language spec, if such usage is "implementation
defined" you can do pretty much whatever you want.  Agressive optimizers
in traditional compilers take advantage of this.

In the case given, though, there's no particular reason why an application
can't grab "currval()" and then use the value returned in the subsequent
query.

On the other hand, heuristics like "if there's no nextval() in the
query, then currval() can be treated as a constant" are very common in
the traditional compiler world, too ...



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