Re: Inadequate executor locking of indexes

On Mon, 11 Feb 2019 at 01:22, Julien Rouhaud <rjuju123(at)gmail(dot)com> wrote:
> The patch is quite straightforward, so I don't have general comments
> on it. However, I think that the idxlockmode initialization is
> problematic: you're using the statement's commandType so this doesn't
> work with CTE. For instance, with this artificial query
>
> WITH s AS (UPDATE test set id = 1 WHERE id =1) select 1;
>
> will take an AccessShareLock on test's index while it should have an
> RowExclusiveLock. I guess that you have to code similar lock upgrade
> logic for the indexes, inspecting the planTree and subplans to find
> the correct command type.

Good catch. I'm a bit stuck on the best way to fix this. So far I can
only think of, either;

1. Adding a new field to RangeTblEntry to indicate the operation type
that's being performed on the relation; or
2. Adding a Bitmapset field to PlannerGlobal that sets the rtable
indexes of RangeTblEntry items that belong to DELETEs and ignore these
when setting resultRelids in finalize_lockmodes().

For #2, the only place I can see to do this is
add_rtes_to_flat_rtable(), which would require either passing the
PlannerInfo into the function, or at least its parse's commandType.

I don't really like either, but don't have any other ideas at the moment.

> > I was also looking at each call site that calls ExecOpenIndices(). I
> > don't think it's great that ExecInitModifyTable() has its own logic to
> > skip calling that function for DELETE. I wondered if it shouldn't
> > somehow depend on what the idxlockmode is set to.
>
> I don't think that it's great either. However for DELETE we shouldn't
> simply call ExecOpenIndices(), but open only the used indexes right?

No, I don't think so. The "used" index(es) will be opened in the scan
node(s). The reason I didn't like it much is that I wanted to keep
the logic for deciding what lock level to use in the planner. The
executor seems to have more knowledge than I think maybe it should.

--
David Rowley http://www.2ndQuadrant.com/
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