Re: [HACKERS] PATCH: enabling parallel execution for cursorsexplicitly (experimental)

On Wed, Nov 1, 2017 at 3:47 AM, Tomas Vondra
<tomas.vondra@2ndquadrant.com> wrote:
> But maybe there's a simpler option - what if we only allow fetches from
> the PARALLEL cursor while the cursor is open? That is, this would work:
>
>     BEGIN;
>     ...
>     DECLARE x PARALLEL CURSOR FOR SELECT * FROM t2 WHERE ...;
>     FETCH 1000 FROM x;
>     FETCH 1000 FROM x;
>     FETCH 1000 FROM x;
>     CLOSE x;
>     ...
>     COMMIT;
>
> but adding any other command between the OPEN/CLOSE commands would fail.
> That should close all the holes with parallel-unsafe stuff, right?

I think that still leaves a fair number of scenarios to consider, and
the error handling by itself seems pretty thorny.  Plus it's kind of a
weird mode and, like Craig, I'm not really sure what it gets you.
Maybe if somebody has the use case where this would help, they should
just do:

CREATE TEMP TABLE x AS SELECT * FROM t2 WHERE ...;
DECLARE x CURSOR FOR SELECT * FROM x;

Since e9baa5e9fa147e00a2466ab2c40eb99c8a700824, that ought to work.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


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