Simplify executor's determination of whether to use parallelism.

Our parallel-mode code only works when we are executing a query
in full, so ExecutePlan must disable parallel mode when it is
asked to do partial execution.  The previous logic for this
involved passing down a flag (variously named execute_once or
run_once) from callers of ExecutorRun or PortalRun.  This is
overcomplicated, and unsurprisingly some of the callers didn't
get it right, since it requires keeping state that not all of
them have handy; not to mention that the requirements for it were
undocumented.  That led to assertion failures in some corner
cases.  The only state we really need for this is the existing
QueryDesc.already_executed flag, so let's just put all the
responsibility in ExecutePlan.  (It could have been done in
ExecutorRun too, leading to a slightly shorter patch -- but if
there's ever more than one caller of ExecutePlan, it seems better
to have this logic in the subroutine than the callers.)

This makes those ExecutorRun/PortalRun parameters unnecessary.
In master it seems okay to just remove them, returning the
API for those functions to what it was before parallelism.
Such an API break is clearly not okay in stable branches,
but for them we can just leave the parameters in place after
documenting that they do nothing.

Per report from Yugo Nagata, who also reviewed and tested
this patch.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/20241206062549.710dc01cf91224809dd6c0e1@sraoss.co.jp

diff --git a/src/backend/executor/execMain.c b/src/backend/executor/execMain.c
index 565262dd27d89807fb16b84a9005aa5ccb49daba..2e0eef7f3de36442bd139cac8ce549d4ebd40c2e 100644 (file)
--- a/src/backend/executor/execMain.c
+++ b/src/backend/executor/execMain.c
@@ -80,14 +80,12 @@ static void InitPlan(QueryDesc *queryDesc, int eflags);
 static void CheckValidRowMarkRel(Relation rel, RowMarkType markType);
 static void ExecPostprocessPlan(EState *estate);
 static void ExecEndPlan(PlanState *planstate, EState *estate);
-static void ExecutePlan(EState *estate, PlanState *planstate,
-                       bool use_parallel_mode,
+static void ExecutePlan(QueryDesc *queryDesc,
                        CmdType operation,
                        bool sendTuples,
                        uint64 numberTuples,
                        ScanDirection direction,
-                       DestReceiver *dest,
-                       bool execute_once);
+                       DestReceiver *dest);
 static bool ExecCheckRTEPerms(RangeTblEntry *rte);
 static bool ExecCheckRTEPermsModified(Oid relOid, Oid userid,
                                      Bitmapset *modifiedCols,
@@ -272,6 +270,9 @@ standard_ExecutorStart(QueryDesc *queryDesc, int eflags)
  *     retrieved tuples, not for instance to those inserted/updated/deleted
  *     by a ModifyTable plan node.
  *
+ *     execute_once is ignored, and is present only to avoid an API break
+ *     in stable branches.
+ *
  *     There is no return value, but output tuples (if any) are sent to
  *     the destination receiver specified in the QueryDesc; and the number
  *     of tuples processed at the top level can be found in
@@ -342,21 +343,12 @@ standard_ExecutorRun(QueryDesc *queryDesc,
     * run plan
     */
    if (!ScanDirectionIsNoMovement(direction))
-   {
-       if (execute_once && queryDesc->already_executed)
-           elog(ERROR, "can't re-execute query flagged for single execution");
-       queryDesc->already_executed = true;
-
-       ExecutePlan(estate,
-                   queryDesc->planstate,
-                   queryDesc->plannedstmt->parallelModeNeeded,
+       ExecutePlan(queryDesc,
                    operation,
                    sendTuples,
                    count,
                    direction,
-                   dest,
-                   execute_once);
-   }
+                   dest);
 
    /*
     * shutdown tuple receiver, if we started it
@@ -1578,22 +1570,19 @@ ExecEndPlan(PlanState *planstate, EState *estate)
  *     moving in the specified direction.
  *
  *     Runs to completion if numberTuples is 0
- *
- * Note: the ctid attribute is a 'junk' attribute that is removed before the
- * user can see it
  * ----------------------------------------------------------------
  */
 static void
-ExecutePlan(EState *estate,
-           PlanState *planstate,
-           bool use_parallel_mode,
+ExecutePlan(QueryDesc *queryDesc,
            CmdType operation,
            bool sendTuples,
            uint64 numberTuples,
            ScanDirection direction,
-           DestReceiver *dest,
-           bool execute_once)
+           DestReceiver *dest)
 {
+   EState     *estate = queryDesc->estate;
+   PlanState  *planstate = queryDesc->planstate;
+   bool        use_parallel_mode;
    TupleTableSlot *slot;
    uint64      current_tuple_count;
 
@@ -1608,11 +1597,17 @@ ExecutePlan(EState *estate,
    estate->es_direction = direction;
 
    /*
-    * If the plan might potentially be executed multiple times, we must force
-    * it to run without parallelism, because we might exit early.
+    * Set up parallel mode if appropriate.
+    *
+    * Parallel mode only supports complete execution of a plan.  If we've
+    * already partially executed it, or if the caller asks us to exit early,
+    * we must force the plan to run without parallelism.
     */
-   if (!execute_once)
+   if (queryDesc->already_executed || numberTuples != 0)
        use_parallel_mode = false;
+   else
+       use_parallel_mode = queryDesc->plannedstmt->parallelModeNeeded;
+   queryDesc->already_executed = true;
 
    estate->es_use_parallel_mode = use_parallel_mode;
    if (use_parallel_mode)

diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 8ad28dce2b7b1d9a22cdba0857d08e7f2dc31617..8c624ee6699c263db1e35852b6e57bcfe6f52a52 100644 (file)
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -1241,7 +1241,7 @@ exec_simple_query(const char *query_string)
        (void) PortalRun(portal,
                         FETCH_ALL,
                         true,  /* always top level */
-                        true,
+                        true,  /* ignored */
                         receiver,
                         receiver,
                         &qc);
@@ -2188,7 +2188,7 @@ exec_execute_message(const char *portal_name, long max_rows)
    completed = PortalRun(portal,
                          max_rows,
                          true, /* always top level */
-                         !execute_is_fetch && max_rows == FETCH_ALL,
+                         true, /* ignored */
                          receiver,
                          receiver,
                          &qc);

diff --git a/src/backend/tcop/pquery.c b/src/backend/tcop/pquery.c
index a8f65ca1658fa4170ae3a1afc056c92a13def8da..3d3f28a9a3cdab367299629102addf0eb3db1642 100644 (file)
--- a/src/backend/tcop/pquery.c
+++ b/src/backend/tcop/pquery.c
@@ -667,6 +667,8 @@ PortalSetResultFormat(Portal portal, int nFormats, int16 *formats)
  * isTopLevel: true if query is being executed at backend "top level"
  * (that is, directly from a client command message)
  *
+ * run_once: ignored, present only to avoid an API break in stable branches.
+ *
  * dest: where to send output of primary (canSetTag) query
  *
  * altdest: where to send output of non-primary queries
@@ -711,10 +713,6 @@ PortalRun(Portal portal, long count, bool isTopLevel, bool run_once,
     */
    MarkPortalActive(portal);
 
-   /* Set run_once flag.  Shouldn't be clear if previously set. */
-   Assert(!portal->run_once || run_once);
-   portal->run_once = run_once;
-
    /*
     * Set up global portal context pointers.
     *
@@ -919,7 +917,7 @@ PortalRunSelect(Portal portal,
        {
            PushActiveSnapshot(queryDesc->snapshot);
            ExecutorRun(queryDesc, direction, (uint64) count,
-                       portal->run_once);
+                       false);
            nprocessed = queryDesc->estate->es_processed;
            PopActiveSnapshot();
        }
@@ -959,7 +957,7 @@ PortalRunSelect(Portal portal,
        {
            PushActiveSnapshot(queryDesc->snapshot);
            ExecutorRun(queryDesc, direction, (uint64) count,
-                       portal->run_once);
+                       false);
            nprocessed = queryDesc->estate->es_processed;
            PopActiveSnapshot();
        }
@@ -1400,9 +1398,6 @@ PortalRunFetch(Portal portal,
     */
    MarkPortalActive(portal);
 
-   /* If supporting FETCH, portal can't be run-once. */
-   Assert(!portal->run_once);
-
    /*
     * Set up global portal context pointers.
     */

diff --git a/src/include/executor/execdesc.h b/src/include/executor/execdesc.h
index b5cead3502d815eda21ea4d2535900b342bdc087..104968f5ba5415d909faf6df0822cc2c76053bad 100644 (file)
--- a/src/include/executor/execdesc.h
+++ b/src/include/executor/execdesc.h
@@ -48,7 +48,7 @@ typedef struct QueryDesc
    EState     *estate;         /* executor's query-wide state */
    PlanState  *planstate;      /* tree of per-plan-node state */
 
-   /* This field is set by ExecutorRun */
+   /* This field is set by ExecutePlan */
    bool        already_executed;   /* true if previously executed */
 
    /* This is always set NULL by the core system, but plugins can change it */

diff --git a/src/include/utils/portal.h b/src/include/utils/portal.h
index a98cc2c095783062d86dfbd61b97090d94defdf0..c5b3e582965aa80543aaafac5ec2cf0d32b70a36 100644 (file)
--- a/src/include/utils/portal.h
+++ b/src/include/utils/portal.h
@@ -144,7 +144,7 @@ typedef struct PortalData
    /* Features/options */
    PortalStrategy strategy;    /* see above */
    int         cursorOptions;  /* DECLARE CURSOR option bits */
-   bool        run_once;       /* portal will only be run once */
+   bool        run_once;       /* unused */
 
    /* Status data */
    PortalStatus status;        /* see above */