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Don't walk into Gather subplans if there's no local scan #56

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Sep 6, 2024
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Don't walk into Gather subplans if there's no local scan #56

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29 changes: 29 additions & 0 deletions expected/parallel.out
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Original file line number Diff line number Diff line change
Expand Up @@ -62,6 +62,35 @@ SELECT count(*) from pg_tracing_consume_spans where span_operation='ExecutorRun'
7
(1 row)

CALL clean_spans();
-- Test leaderless parallel query
set parallel_setup_cost=0;
set parallel_tuple_cost=0;
set min_parallel_table_scan_size=0;
set max_parallel_workers_per_gather=2;
set parallel_leader_participation=false;
/*dddbs='postgres.db',traceparent='00-00000000000000000000000000000001-0000000000000001-01'*/ select 1 from pg_class limit 1;
?column?
----------
1
(1 row)

SELECT span_type, span_operation, lvl FROM peek_ordered_spans where trace_id='00000000000000000000000000000001' ORDER BY lvl, span_operation;
span_type | span_operation | lvl
--------------+-----------------------------------+-----
Select query | select $1 from pg_class limit $2; | 1
ExecutorRun | ExecutorRun | 2
Planner | Planner | 2
Limit | Limit | 3
Select query | Worker 0 | 3
Select query | Worker 1 | 3
ExecutorRun | ExecutorRun | 4
ExecutorRun | ExecutorRun | 4
Gather | Gather | 4
SeqScan | SeqScan on pg_class | 5
SeqScan | SeqScan on pg_class | 5
(11 rows)

-- Cleanup
CALL clean_spans();
CALL reset_settings();
4 changes: 4 additions & 0 deletions expected/setup.out
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Expand Up @@ -17,6 +17,10 @@ AS $$
SET pg_tracing.caller_sample_rate TO DEFAULT;
SET pg_tracing.track_utility TO DEFAULT;
SET pg_tracing.max_parameter_size TO DEFAULT;
SET parallel_setup_cost TO DEFAULT;
SET parallel_tuple_cost TO DEFAULT;
SET min_parallel_table_scan_size TO DEFAULT;
SET max_parallel_workers_per_gather TO DEFAULT;
$$;
CREATE OR REPLACE PROCEDURE reset_pg_tracing_test_table() AS $$
BEGIN
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11 changes: 11 additions & 0 deletions sql/parallel.sql
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Expand Up @@ -30,6 +30,17 @@ SELECT trace_id from pg_tracing_peek_spans group by trace_id;

-- Check number of executor spans
SELECT count(*) from pg_tracing_consume_spans where span_operation='ExecutorRun';
CALL clean_spans();

-- Test leaderless parallel query
set parallel_setup_cost=0;
set parallel_tuple_cost=0;
set min_parallel_table_scan_size=0;
set max_parallel_workers_per_gather=2;
set parallel_leader_participation=false;
/*dddbs='postgres.db',traceparent='00-00000000000000000000000000000001-0000000000000001-01'*/ select 1 from pg_class limit 1;

SELECT span_type, span_operation, lvl FROM peek_ordered_spans where trace_id='00000000000000000000000000000001' ORDER BY lvl, span_operation;

-- Cleanup
CALL clean_spans();
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4 changes: 4 additions & 0 deletions sql/setup.sql
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Expand Up @@ -19,6 +19,10 @@ AS $$
SET pg_tracing.caller_sample_rate TO DEFAULT;
SET pg_tracing.track_utility TO DEFAULT;
SET pg_tracing.max_parameter_size TO DEFAULT;
SET parallel_setup_cost TO DEFAULT;
SET parallel_tuple_cost TO DEFAULT;
SET min_parallel_table_scan_size TO DEFAULT;
SET max_parallel_workers_per_gather TO DEFAULT;
$$;

CREATE OR REPLACE PROCEDURE reset_pg_tracing_test_table() AS $$
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37 changes: 35 additions & 2 deletions src/pg_tracing_planstate.c
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Expand Up @@ -279,6 +279,36 @@ create_spans_from_custom_scan(CustomScanState *css, planstateTraceContext * plan
return last_end;
}

/*
* Check if the subplan will run or not
*
* With parallel queries, if leader participation is disabled, the leader won't execute the subplans
*/
static bool
is_subplan_executed(PlanState *planstate)
{
if (planstate == NULL)
return false;

switch (nodeTag(planstate))
{
case T_GatherState:
{
GatherState *splanstate = (GatherState *) planstate;

return splanstate->need_to_scan_locally;
}
case T_GatherMerge:
{
GatherMergeState *splanstate = (GatherMergeState *) planstate;

return splanstate->need_to_scan_locally;
}
default:
return true;
}
}

/*
* Walk through the planstate tree generating a node span for each node.
*/
Expand All @@ -295,6 +325,7 @@ create_spans_from_planstate(PlanState *planstate, planstateTraceContext * planst
TimestampTz span_end;
TimestampTz child_end = 0;
bool haschildren = false;
bool subplan_executed = true;

/* The node was never executed, skip it */
if (planstate->instrument == NULL)
Expand Down Expand Up @@ -379,11 +410,13 @@ create_spans_from_planstate(PlanState *planstate, planstateTraceContext * planst
if (haschildren && planstateTraceContext->deparse_ctx)
planstateTraceContext->ancestors = lcons(planstate->plan, planstateTraceContext->ancestors);

/* We only need to walk the subplans if they are executed */
subplan_executed = is_subplan_executed(planstate);
/* Walk the outerplan */
if (outerPlanState(planstate))
if (outerPlanState(planstate) && subplan_executed)
create_spans_from_planstate(outerPlanState(planstate), planstateTraceContext, span_id, query_id, span_start, root_end, latest_end);
/* Walk the innerplan */
if (innerPlanState(planstate))
if (innerPlanState(planstate) && subplan_executed)
create_spans_from_planstate(innerPlanState(planstate), planstateTraceContext, span_id, query_id, span_start, root_end, latest_end);

/* Handle init plans */
Expand Down