Fix parallel hash span creation

Author: blast-hardcheese

expected/parallel.out

@@ -70,6 +70,15 @@ set parallel_setup_cost=0;
 set parallel_tuple_cost=0;
 set min_parallel_table_scan_size=0;
 set max_parallel_workers_per_gather=2;
+-- Test a sampled parallel hash join. Parallel Hash nodes may have
+-- instrumentation even when their child was executed by another worker.
+set enable_nestloop=false;
+set enable_mergejoin=false;
+/*dddbs='postgres.db',traceparent='00-00000000000000000000000000000001-0000000000000005-01'*/ select count(*) from pg_tracing_test r join pg_tracing_test s using (a) \gset
+\echo :count
+10000
+CALL clean_spans();
+-- Test leaderless parallel query
 set parallel_leader_participation=false;
 /*dddbs='postgres.db',traceparent='00-00000000000000000000000000000001-0000000000000001-01'*/ select 1 from pg_class limit 1;
  ?column? 

regress/16/expected/parallel.out

@@ -70,6 +70,15 @@ set parallel_setup_cost=0;
 set parallel_tuple_cost=0;
 set min_parallel_table_scan_size=0;
 set max_parallel_workers_per_gather=2;
+-- Test a sampled parallel hash join. Parallel Hash nodes may have
+-- instrumentation even when their child was executed by another worker.
+set enable_nestloop=false;
+set enable_mergejoin=false;
+/*dddbs='postgres.db',traceparent='00-00000000000000000000000000000001-0000000000000005-01'*/ select count(*) from pg_tracing_test r join pg_tracing_test s using (a) \gset
+\echo :count
+10000
+CALL clean_spans();
+-- Test leaderless parallel query
 set parallel_leader_participation=false;
 /*dddbs='postgres.db',traceparent='00-00000000000000000000000000000001-0000000000000001-01'*/ select 1 from pg_class limit 1;
  ?column? 

regress/17/expected/parallel.out

@@ -70,6 +70,15 @@ set parallel_setup_cost=0;
 set parallel_tuple_cost=0;
 set min_parallel_table_scan_size=0;
 set max_parallel_workers_per_gather=2;
+-- Test a sampled parallel hash join. Parallel Hash nodes may have
+-- instrumentation even when their child was executed by another worker.
+set enable_nestloop=false;
+set enable_mergejoin=false;
+/*dddbs='postgres.db',traceparent='00-00000000000000000000000000000001-0000000000000005-01'*/ select count(*) from pg_tracing_test r join pg_tracing_test s using (a) \gset
+\echo :count
+10000
+CALL clean_spans();
+-- Test leaderless parallel query
 set parallel_leader_participation=false;
 /*dddbs='postgres.db',traceparent='00-00000000000000000000000000000001-0000000000000001-01'*/ select 1 from pg_class limit 1;
  ?column? 

regress/18/expected/parallel.out

@@ -70,6 +70,15 @@ set parallel_setup_cost=0;
 set parallel_tuple_cost=0;
 set min_parallel_table_scan_size=0;
 set max_parallel_workers_per_gather=2;
+-- Test a sampled parallel hash join. Parallel Hash nodes may have
+-- instrumentation even when their child was executed by another worker.
+set enable_nestloop=false;
+set enable_mergejoin=false;
+/*dddbs='postgres.db',traceparent='00-00000000000000000000000000000001-0000000000000005-01'*/ select count(*) from pg_tracing_test r join pg_tracing_test s using (a) \gset
+\echo :count
+10000
+CALL clean_spans();
+-- Test leaderless parallel query
 set parallel_leader_participation=false;
 /*dddbs='postgres.db',traceparent='00-00000000000000000000000000000001-0000000000000001-01'*/ select 1 from pg_class limit 1;
  ?column? 

sql/parallel.sql

@@ -41,6 +41,16 @@ set parallel_setup_cost=0;
 set parallel_tuple_cost=0;
 set min_parallel_table_scan_size=0;
 set max_parallel_workers_per_gather=2;
+
+-- Test a sampled parallel hash join. Parallel Hash nodes may have
+-- instrumentation even when their child was executed by another worker.
+set enable_nestloop=false;
+set enable_mergejoin=false;
+/*dddbs='postgres.db',traceparent='00-00000000000000000000000000000001-0000000000000005-01'*/ select count(*) from pg_tracing_test r join pg_tracing_test s using (a) \gset
+\echo :count
+CALL clean_spans();
+
+-- Test leaderless parallel query
 set parallel_leader_participation=false;
 /*dddbs='postgres.db',traceparent='00-00000000000000000000000000000001-0000000000000001-01'*/ select 1 from pg_class limit 1;
 

src/pg_tracing_planstate.c

@@ -395,10 +395,12 @@ create_spans_from_planstate(PlanState *planstate, planstateTraceContext * planst
 			span_id = generate_rnd_uint64();
 			break;
 		case T_Hash:
-			/* For hash node, use the child's start */
+			/* The child may be executed by another parallel worker */
 			traced_planstate = get_traced_planstate(outerPlanState(planstate));
-			Assert(traced_planstate != NULL);
-			span_start = traced_planstate->node_start;
+			if (traced_planstate != NULL)
+				span_start = traced_planstate->node_start;
+			else
+				span_start = parent_start;
 
 			/*
 			 * We still need to generate a dedicated span_id since