Re: Management of simple_eval_estate for plpgsql DO blocks

On 08/14/2015 12:42 PM, Tom Lane wrote:
> In commit 0fc94a5ba I wrote:
>
> + * ... It's okay to update the [ session-wide ]
> + * hash table with the new tree because all plpgsql functions within a
> + * given transaction share the same simple_eval_estate.
>
> Um. Well, that's true for actual functions, but plpgsql DO blocks use
> their own private simple_eval_estate. That means that after a DO block
> runs, the cast_hash contains dangling pointers to expression eval state
> trees, which a subsequent plpgsql execution in the same transaction will
> think are still valid. Ooops. (See bug #13571.)
>
> The simplest fix for this would be to give up on the idea that DO blocks
> use private simple_eval_estates, and make them use the shared one.
> However, that would result in intra-transaction memory bloat for
> transactions executing large numbers of DO blocks; see commit c7b849a89,
> which installed that arrangement to begin with. Since that change was
> based on a user complaint, this answer doesn't seem appetizing.
>
> Or we could try to use the shared simple_eval_estate for CAST expressions
> even within DO blocks, but I'm afraid that would break things in subtle
> ways. We need to do actual execution in the block's own eval_estate,
> or we will have problems with leakage of pass-by-reference cast results
> because exec_eval_cleanup() won't know to clean them up. It's possible
> that we could get away with putting the expression state tree into the
> shared simple_eval_estate's per-query memory and then executing it with
> the block's private simple_eval_estate, but I'm afraid there are probably
> places in execQual and/or C functions that suppose that the expression
> state tree is in the estate's per-query memory. (That is, I doubt that
> we're totally consistent about whether we use fcinfo->flinfo->fn_mcxt or
> econtext->ecxt_per_query_memory for long-lived data, in which case an
> arrangement like this could lead to dangling pointers.)
>
> Or we could change things so that DO blocks use private cast_hash
> hashtables along with their private simple_eval_estates. This would
> give up some efficiency (since a DO block would then always need to do
> its own cast lookups) but it would be a simple and reliable fix.
>
> I'm kind of inclined to go with the last choice, but I wonder if anyone
> wants to argue differently, or sees another feasible solution.
>
>

+1 for the last unless someone comes up with something better. Is it
going to involve a huge hit anyway? It seems unlikely to matter that much.

cheers

andrew