Hello Pavel,

Sometimes I have to solve the result types of some query. It is invisible
in psql.

Indeed. My 0.02€ about this patch:

About the feature:

It looks useful to allow to show the resulting types of a query.

About the code:

Patch applies cleanly and compiles.

I'm afraid that re-using the "results" variable multiple times results in memory leaks... ISTM that new variables should be used when going down the nested conditions, and all cleanup should be done where and when necessary.

Also, maybe it would be better if the statement is cleaned up server side at the end of the execution. Not sure how to achieve that, though, libpq seems to lack the relevant function:-(

  """although there is no libpq function for deleting a prepared
  statement, the SQL DEALLOCATE statement can be used for that purpose."""

Hmmm... I have not found how to use DEALLOCATE to cleanup an unnamed statement, it does not allow a "zero-length" name. Maybe it could be extended somehow, or a function could be provided for the purpose, eg
by passing a NULL query to PQprepare...

Resetting "gdesc flag" could be done only when the flag was true, at the end of the if, rather than on each query.

I understand that the second level query is to retrieve the type names in place of the Oid returned by QPftype?

The pg_malloc length computation looks a little bit arbitrary. Would it make sense to use PQescapeLiteral instead?

About the documentation:

I would suggest some rewording, maybe:

"Show the description of the result of the current query buffer without actually executing it, by considering it a prepared statement."

About tests:

There should be some non-regression tests added, maybe something like:

    NULL AS zero,
    1 AS one,
    2.0 AS two,
    'three' AS three,
    $1 AS four,
    -- ...

And also case which trigger an error, eg:

  SELECT $1 AS unknown_type \gdesc
  SELECT 1 + \gdesc

Some fun:

  EXECUTE test \gdesc

I'm unsure about some error messages, but it may be unrelated to this patch:

 calvin=# SELECT '1 km'::unit AS foo, $2 as boo \gdesc
 ERROR:  could not determine data type of parameter $1

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