Subject: Re: COPY FROM WHEN condition
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Andres Freund <andres@anarazel.de>
Cc: Surafel Temesgen <surafel3000@gmail.com>, alvherre@2ndquadrant.com,
 Adam Berlin <berlin.ab@gmail.com>, pgsql-hackers@lists.postgresql.org
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Message-ID: <f66a0132-c707-0a3e-fb53-5880ec06bf28@2ndquadrant.com>
Date: Mon, 21 Jan 2019 16:22:11 +0100
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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
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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diff --git a/src/backend/commands/copy.c b/src/backend/commands/copy.c
index 05d53f96f6..e55b992857 100644
--- a/src/backend/commands/copy.c
+++ b/src/backend/commands/copy.c
@@ -2612,8 +2612,7 @@ CopyFrom(CopyState cstate)
 		 */
 		insertMethod = CIM_SINGLE;
 	}
-	else if (cstate->whereClause != NULL ||
-			 contain_volatile_functions(cstate->whereClause))
+	else if (contain_volatile_functions(cstate->whereClause))
 	{
 		/*
 		 * Can't support multi-inserts if there are any volatile funcation

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