[yugabyte#27535] YSQL: Fix PgGate's parallel requests

Summary:
Handle the case when all the parallel requests are pruned.
In PgGate requests may take bounds from different sources, like target
tablet bounds, conditions on the primary key, parallel ranges. When we
apply multiple bounds to a request, they may result in empty range.
We prune empty range requests. There are rare, but valid cases when all
tablet requests are pruned. PgGate did not check that and proceeded with
sending zero requests, which triggered assertion failure down the road.
In this diff we check if we have any requests before we send, and skip
sending if we don't.
It is now a valid case now if PgDocOp was executed, but did not sent any
requests. It considers that as the end of the execution not as a
failure.
Jira: DB-17096

Test Plan: ./yb_build.sh --java-test 'org.yb.pgsql.TestPgRegressIndex#schedule'

Reviewers: jason, telgersma

Reviewed By: jason

Subscribers: yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D44664

Author: andrei-mart

src/postgres/src/test/regress/expected/yb.orig.index_scan.out

@@ -1651,6 +1651,19 @@ SELECT * FROM pk_range_int_asc WHERE (r1, r2, r3) <= (1,1,5) AND (r1,r2,r3) < (1
 ----+----+----+---
 (0 rows)
 
+EXPLAIN (COSTS OFF, TIMING OFF, SUMMARY OFF, ANALYZE) SELECT * FROM pk_range_int_asc WHERE (r1, r2, r3) <= (1,1,5) AND (r1, r2, r3) > (1,2,3) AND v IS NOT NULL;
+                                       QUERY PLAN                                       
+----------------------------------------------------------------------------------------
+ Index Scan using pk_range_int_asc_pkey on pk_range_int_asc (actual rows=0 loops=1)
+   Index Cond: ((ROW(r1, r2, r3) <= ROW(1, 1, 5)) AND (ROW(r1, r2, r3) > ROW(1, 2, 3)))
+   Storage Filter: (v IS NOT NULL)
+(3 rows)
+
+SELECT * FROM pk_range_int_asc WHERE (r1, r2, r3) <= (1,1,5) AND (r1, r2, r3) > (1,2,3) AND v IS NOT NULL;
+ r1 | r2 | r3 | v 
+----+----+----+---
+(0 rows)
+
 EXPLAIN (COSTS OFF, TIMING OFF, SUMMARY OFF, ANALYZE) SELECT * FROM pk_range_int_asc WHERE (r1, r3) <= (1,3) AND (r1,r2) < (1,3) AND (r1,r2) >= (1,2);
                                                QUERY PLAN                                                
 ---------------------------------------------------------------------------------------------------------

src/postgres/src/test/regress/sql/yb.orig.index_scan.sql

@@ -287,6 +287,8 @@ EXPLAIN (COSTS OFF, TIMING OFF, SUMMARY OFF, ANALYZE) SELECT * FROM pk_range_int
 SELECT * FROM pk_range_int_asc WHERE (r1, r2, r3) <= (1,6,5) AND (r1,r2,r3) < (1,6,5);
 EXPLAIN (COSTS OFF, TIMING OFF, SUMMARY OFF, ANALYZE) SELECT * FROM pk_range_int_asc WHERE (r1, r2, r3) <= (1,1,5) AND (r1,r2,r3) < (1,2,4) AND (r1,r2,r3) > (1,2,3) AND (r1,r2,r3) < (1,2,4) AND (r1,r2,r3) >= (1,2,3);
 SELECT * FROM pk_range_int_asc WHERE (r1, r2, r3) <= (1,1,5) AND (r1,r2,r3) < (1,2,4) AND (r1,r2,r3) > (1,2,3) AND (r1,r2,r3) < (1,2,4) AND (r1,r2,r3) >= (1,2,3);
+EXPLAIN (COSTS OFF, TIMING OFF, SUMMARY OFF, ANALYZE) SELECT * FROM pk_range_int_asc WHERE (r1, r2, r3) <= (1,1,5) AND (r1, r2, r3) > (1,2,3) AND v IS NOT NULL;
+SELECT * FROM pk_range_int_asc WHERE (r1, r2, r3) <= (1,1,5) AND (r1, r2, r3) > (1,2,3) AND v IS NOT NULL;
 EXPLAIN (COSTS OFF, TIMING OFF, SUMMARY OFF, ANALYZE) SELECT * FROM pk_range_int_asc WHERE (r1, r3) <= (1,3) AND (r1,r2) < (1,3) AND (r1,r2) >= (1,2);
 SELECT * FROM pk_range_int_asc WHERE (r1, r3) <= (1,3) AND (r1,r2) < (1,3) AND (r1,r2) >= (1,2);
 EXPLAIN (COSTS OFF, TIMING OFF, SUMMARY OFF, ANALYZE) SELECT * FROM pk_range_int_asc WHERE (r1, r3) <= (1,3) AND (r1,r2) < (1,3) AND (r1,r2) >= (1,2) AND (r1,r2,r3) = (1,2,3);

src/yb/yql/pggate/pg_dml_read.cc

@@ -466,10 +466,9 @@ Status PgDmlRead::Exec(const YbcPgExecParameters* exec_params) {
     // No ybctids are provided. Instruct "doc_op_" to abandon the execution and not querying
     // any data from tablet server.
     doc_op_->AbandonExecution();
-  } else {
-    RSTATUS_DCHECK_EQ(
-        VERIFY_RESULT(doc_op_->Execute()),
-        RequestSent::kTrue, IllegalState, "YSQL read operation was not sent");
+  } else if (VERIFY_RESULT(doc_op_->Execute()) != RequestSent::kTrue) {
+    // Requests weren't sent, there is no data to fetch
+    doc_op_->AbandonExecution();
   }
 
   return Status::OK();

src/yb/yql/pggate/pg_doc_op.cc

@@ -372,12 +372,14 @@ Status PgDocOp::SendRequestImpl(ForceNonBufferable force_non_bufferable) {
   // Populate collected information into protobuf requests before sending to DocDB.
   RETURN_NOT_OK(CreateRequests());
 
-  // Send at most "parallelism_level_" number of requests at one time.
-  size_t send_count = std::min(parallelism_level_, active_op_count_);
-  VLOG(1) << "Number of operations to send: " << send_count;
-  response_ = VERIFY_RESULT(sender_(
-      pg_session_.get(), pgsql_ops_.data(), send_count, *table_,
-      HybridTime::FromPB(GetInTxnLimitHt()), force_non_bufferable, IsForWritePgDoc(IsWrite())));
+  if (active_op_count_ > 0) {
+    // Send at most "parallelism_level_" number of requests at one time.
+    size_t send_count = std::min(parallelism_level_, active_op_count_);
+    VLOG(1) << "Number of operations to send: " << send_count;
+    response_ = VERIFY_RESULT(sender_(
+        pg_session_.get(), pgsql_ops_.data(), send_count, *table_,
+        HybridTime::FromPB(GetInTxnLimitHt()), force_non_bufferable, IsForWritePgDoc(IsWrite())));
+  }
   return Status::OK();
 }