Re: Differentiate MERGE queries with different structures

On 2022-Sep-26, Julien Rouhaud wrote:

> On Mon, Sep 26, 2022 at 03:12:46PM +0900, bt22nakamorit wrote:

> > I attached a patch file that adds information about MERGE queries on the
> > documentation of pg_stat_statements, and lines of code that helps with the
> > calculation of queryid hash value to differentiate MERGE queries.
> > Any kind of feedback is appreciated.
>
> I didn't test the patch (and never looked at MERGE implementation either), but
> I'm wondering if MergeAction->matched and MergeAction->override should be
> jumbled too?

->matched distinguishes these two queries:

MERGE INTO foo USING bar ON (something)
WHEN MATCHED THEN DO NOTHING;
MERGE INTO foo USING bar ON (something)
WHEN NOT MATCHED THEN DO NOTHING;

because only DO NOTHING can be used with both MATCHED and NOT MATCHED,
though on the whole the distinction seems pointless. However I think if
you sent both these queries and got a single pgss entry with the text of
one of them and not the other, you're going to be confused about where
the other went. So +1 for jumbling it too.

->overriding is used in OVERRIDING SYSTEM VALUES (used for GENERATED
columns). I don't think there's any case where it's interesting
currently: if you specify the column it's going to be in the column list
(which does affect the query ID).

> Also, the patch should contain some extra tests to fully cover MERGE
> jumbling.

Agreed. I struggle to find a balance between not having anything and
going overboard, but I decided to add different for the different things
that should be jumbled, so that they would all appear in the view.

I moved the code around; instead of adding it at the end of the switch,
I did what the comment says, which is to mirror expression_tree_walker.
This made me realize that the latter is using the wrong order for fields
according to the struct definition, so I flipped that also.

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