Re: COPY FROM WHEN condition - Mailing list pgsql-hackers

On 1/21/19 4:33 AM, Tomas Vondra wrote:
> 
> 
> On 1/21/19 3:12 AM, Andres Freund wrote:
>> On 2019-01-20 18:08:05 -0800, Andres Freund wrote:
>>> On 2019-01-20 21:00:21 -0500, Tomas Vondra wrote:
>>>>
>>>>
>>>> On 1/20/19 8:24 PM, Andres Freund wrote:
>>>>> Hi,
>>>>>
>>>>> On 2019-01-20 00:24:05 +0100, Tomas Vondra wrote:
>>>>>> On 1/14/19 10:25 PM, Tomas Vondra wrote:
>>>>>>> On 12/13/18 8:09 AM, Surafel Temesgen wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On Wed, Dec 12, 2018 at 9:28 PM Tomas Vondra
>>>>>>>> <tomas.vondra@2ndquadrant.com <mailto:tomas.vondra@2ndquadrant.com>> wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>>      Can you also update the docs to mention that the functions called from
>>>>>>>>      the WHERE clause does not see effects of the COPY itself?
>>>>>>>>
>>>>>>>>
>>>>>>>> /Of course, i  also add same comment to insertion method selection
>>>>>>>> /
>>>>>>>
>>>>>>> FWIW I've marked this as RFC and plan to get it committed this week.
>>>>>>>
>>>>>>
>>>>>> Pushed, thanks for the patch.
>>>>>
>>>>> While rebasing the pluggable storage patch ontop of this I noticed that
>>>>> the qual appears to be evaluated in query context. Isn't that a bad
>>>>> idea? ISMT it should have been evaluated a few lines above, before the:
>>>>>
>>>>>         /* Triggers and stuff need to be invoked in query context. */
>>>>>         MemoryContextSwitchTo(oldcontext);
>>>>>
>>>>> Yes, that'd require moving the ExecStoreHeapTuple(), but that seems ok?
>>>>>
>>>>
>>>> Yes, I agree. It's a bit too late for me to hack and push stuff, but I'll
>>>> fix that tomorrow.
>>>
>>> NP. On second thought, the problem is probably smaller than I thought at
>>> first, because ExecQual() switches to the econtext's per-tuple memory
>>> context. But it's only reset once for each batch, so there's some
>>> wastage. At least worth a comment.
>>
>> I'm tired, but perhaps its actually worse - what's being reset currently
>> is the ESTate's per-tuple context:
>>
>>         if (nBufferedTuples == 0)
>>         {
>>             /*
>>              * Reset the per-tuple exprcontext. We can only do this if the
>>              * tuple buffer is empty. (Calling the context the per-tuple
>>              * memory context is a bit of a misnomer now.)
>>              */
>>             ResetPerTupleExprContext(estate);
>>         }
>>
>> but the quals are evaluated in the ExprContext's:
>>
>> ExecQual(ExprState *state, ExprContext *econtext)
>> ...
>>     ret = ExecEvalExprSwitchContext(state, econtext, &isnull);
>>
>>
>> which is created with:
>>
>> /* Get an EState's per-output-tuple exprcontext, making it if first use */
>> #define GetPerTupleExprContext(estate) \
>>     ((estate)->es_per_tuple_exprcontext ? \
>>      (estate)->es_per_tuple_exprcontext : \
>>      MakePerTupleExprContext(estate))
>>
>> and creates its own context:
>>     /*
>>      * Create working memory for expression evaluation in this context.
>>      */
>>     econtext->ecxt_per_tuple_memory =
>>         AllocSetContextCreate(estate->es_query_cxt,
>>                               "ExprContext",
>>                               ALLOCSET_DEFAULT_SIZES);
>>
>> so this is currently just never reset.
> 
> Actually, no. The ResetPerTupleExprContext boils down to
> 
>     MemoryContextReset((econtext)->ecxt_per_tuple_memory)
> 
> and ExecEvalExprSwitchContext does this
> 
>     MemoryContextSwitchTo(econtext->ecxt_per_tuple_memory);
> 
> So it's resetting the right context, although only on batch boundary.
> But now I see 31f38174 does this:
> 
>     else if (cstate->whereClause != NULL ||
>              contain_volatile_functions(cstate->whereClause))
>     {
>         ...
>         insertMethod = CIM_SINGLE;
>     }
> 
> so it does not do batching with WHERE. But the condition seems wrong, I
> guess it should be && instead of ||. Will investigate in the morning.
> 

I think the condition can be just

    if (contain_volatile_functions(cstate->whereClause)) { ... }

Per the attached patch. Surafel, do you agree?


>> Seems just using ExecQualAndReset() ought to be sufficient?
>>
> 
> That may still be the right thing to do.
> 

Actually, no, because that would reset the context far too early (and
it's easy to trigger segfaults). So the reset would have to happen after
processing the row, not this early.

But I think the current behavior is actually OK, as it matches what we
do for defexprs. And the comment before ResetPerTupleExprContext says this:

    /*
     * Reset the per-tuple exprcontext. We can only do this if the
     * tuple buffer is empty. (Calling the context the per-tuple
     * memory context is a bit of a misnomer now.)
     */

So the per-tuple context is not quite per-tuple anyway. Sure, we might
rework that but I don't think that's an issue in this patch.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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