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Force NO SCROLL for plpgsql's implicit cursors.

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Further thought about bug #17050 suggests that it's a good idea
to use CURSOR_OPT_NO_SCROLL for the implicit cursor opened by
a plpgsql FOR-over-query loop.  This ensures that, if somebody
commits inside the loop, PersistHoldablePortal won't try to
rewind and re-read the cursor.  While we'd have selected NO_SCROLL
anyway if FOR UPDATE/SHARE appears in the query, there are other
hazards with volatile functions; and in any case, it's silly to
expend effort storing rows that we know for certain won't be needed.

(While here, improve the comment in exec_run_select, which was a bit
confused about the rationale for when we can use parallel mode.
Cursor operations aren't a hazard for nameless portals.)

This wasn't an issue until v11, which introduced the possibility
of persisting such cursors.  Hence, back-patch to v11.

Per bug #17050 from Алексей Булгаков.

Discussion: https://postgr.es/m/17050-f77aa827dc85247c@postgresql.org
This commit is contained in:
Tom Lane 2021-06-08 18:40:06 -04:00
parent c1fd756fd2
commit c5b2818413
1 changed files with 14 additions and 7 deletions
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21
src/pl/plpgsql/src/pl_exec.c
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@ -4558,7 +4558,7 @@ exec_stmt_dynfors(PLpgSQL_execstate *estate, PLpgSQL_stmt_dynfors *stmt)
int rc;
portal = exec_dynquery_with_params(estate, stmt->query, stmt->params,
NULL, 0);
NULL, CURSOR_OPT_NO_SCROLL);
/*
* Execute the loop
@ -5903,14 +5903,21 @@ exec_run_select(PLpgSQL_execstate *estate,
* On the first call for this expression generate the plan.
*
* If we don't need to return a portal, then we're just going to execute
* the query once, which means it's OK to use a parallel plan, even if the
* number of rows being fetched is limited. If we do need to return a
* portal, the caller might do cursor operations, which parallel query
* can't support.
* the query immediately, which means it's OK to use a parallel plan, even
* if the number of rows being fetched is limited. If we do need to
* return a portal (i.e., this is for a FOR loop), the user's code might
* invoke additional operations inside the FOR loop, making parallel query
* unsafe. In any case, we don't expect any cursor operations to be done,
* so specify NO_SCROLL for efficiency and semantic safety.
*/
if (expr->plan == NULL)
exec_prepare_plan(estate, expr,
portalP == NULL ? CURSOR_OPT_PARALLEL_OK : 0, true);
{
int cursorOptions = CURSOR_OPT_NO_SCROLL;
if (portalP == NULL)
cursorOptions |= CURSOR_OPT_PARALLEL_OK;
exec_prepare_plan(estate, expr, cursorOptions, true);
}
/*
* Set up ParamListInfo to pass to executor