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Re: Access to Row ID information in Functions

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Paul Ramsey" <pramsey(at)cleverelephant(dot)ca>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Access to Row ID information in Functions
Date: 2008-04-01 21:37:43
Message-ID: 14938.1207085863@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
"Paul Ramsey" <pramsey(at)cleverelephant(dot)ca> writes:
> The "optimized" form gets cached and retrieved from a memory context.
> Each time the function is run within a statement it checks the cache,
> and sees if one of its arguments are the same as the last time around.
> If so, it uses the prepared version of that argument. If not, it
> builds a new prepared version and caches that.

> The key here is being able to check the identify of the arguments...
> is this argument A the same as the one we processed last time? One way
> is to do a memcmp.  But it seems likely that PgSQL knows exactly
> whether it is running a nested loop, or a literal, and could tell
> somehow that argument A is the same with each call.

Not really.  Certainly there's no way that that information would
propagate into function calls.

In the special case where your argument is a literal constant, I think
there is enough information available to detect that that's the case
(look at get_fn_expr_argtype).  But if it's not, there's no very good
way to know whether it's the same as last time.

Perhaps it would be worth changing your on-disk storage format to allow
cheaper checking?  For instance include a hash value.

			regards, tom lane

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