DECLARE CURSOR must not contain data-modifying statements in WITH

Hi all,

Whats the reason for disallowing cursors on wCTEs? I am not sure I can follow
the comment:

/*
* We also disallow data-modifying WITH in a cursor. (This could be
* allowed, but the semantics of when the updates occur might be
* surprising.)
*/
if (result->hasModifyingCTE)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("DECLARE CURSOR must not contain data-modifying
statements in WITH")));

Given that cursors are about the only sensible way to return larger amounts of
data, that behaviour reduces the usefulness of wCTEs a bit.

Whats the exact cause of concern here? I personally don't think there is a
problem documenting that you should fetch the cursor fully before relying on
the updated tables to be in a sensible state. But that may be just me.

Thanks,

Andres