Re: machine-readable explain output v4

On Sun, Aug 2, 2009 at 7:57 PM, Tom Lane<tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>>>> The reason for this regression is that Tom asked me to change
>>>> ExplainStmt to just carry a list of nodes and to do all the parsing in
>>>> ExplainQuery.  Unfortunately, the TupleDesc is constructed by
>>>> ExplainResultDesc() which can't trivially be changed to take an
>>>> ExplainState, because UtilityTupleDescriptor() also wants to call it.
>>>> We could possibly fix this by a hack similar to the one we already
>>>> added to GetCommandLogLevel(), but I haven't done that here.
>
> I don't see anything particularly wrong with having ExplainResultDesc
> do the same kind of thing GetCommandLogLevel is doing.

After I did this, I thought it would be useful to add a regression
test to make sure that it is doing the right thing.  So I came up with
this:

CREATE OR REPLACE FUNCTION test_explain_format(text) RETURNS text AS $$
DECLARE       x RECORD;
BEGIN       EXECUTE 'explain (format ' || $1 || ') select 1' INTO x;       RETURN pg_typeof(x."QUERY PLAN");
END
$$ LANGUAGE plpgsql;

This works the first time you run it in a particular session, but then
if change $1 so as to get a different answer, it fails:

rhaas=# select test_explain_format('text');test_explain_format
---------------------text
(1 row)

rhaas=# select test_explain_format('xml');
ERROR:  type of "x.QUERY PLAN" does not match that when preparing the plan
CONTEXT:  PL/pgSQL function "test_explain_format" line 5 at RETURN
rhaas=# discard
ALL    PLANS  TEMP
rhaas=# discard plans;
DISCARD PLANS
rhaas=# select test_explain_format('xml');
ERROR:  type of "x.QUERY PLAN" does not match that when preparing the plan
CONTEXT:  PL/pgSQL function "test_explain_format" line 5 at RETURN
rhaas=# discard all;
DISCARD ALL
rhaas=# select test_explain_format('xml');
ERROR:  type of "x.QUERY PLAN" does not match that when preparing the plan
CONTEXT:  PL/pgSQL function "test_explain_format" line 5 at RETURN
rhaas=#

If I quit psql and start back up again, then it works:

rhaas=# select test_explain_format('xml');test_explain_format
---------------------xml
(1 row)

So I guess that leads me to -

(1) How do I make this work?
(2) Is it worth making this work?

...Robert