Error out if SKIP LOCKED and WITH TIES are both specified

Both bugs #16676[1] and #17141[2] illustrate that the combination of
SKIP LOCKED and FETCH FIRST WITH TIES break expectations when it comes
to rows returned to other sessions accessing the same row.  Since this
situation is detectable from the syntax and hard to fix otherwise,
forbid for now, with the potential to fix in the future.

[1] https://postgr.es/m/[email protected]
[2] https://postgr.es/m/[email protected]

Backpatch-through: 13, where WITH TIES was introduced
Author: David Christensen <[email protected]>
Discussion: https://postgr.es/m/CAOxo6XLPccCKru3xPMaYDpa+AXyPeWFs+SskrrL+HKwDjJnLhg@mail.gmail.com

5 files changed, 29 insertions, 2 deletions

diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index fa676b1698..16bbab52c3 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -1515,7 +1515,8 @@ FETCH { FIRST | NEXT } [ <replaceable class="parameter">count</replaceable> ] {
     The <literal>WITH TIES</literal> option is used to return any additional
     rows that tie for the last place in the result set according to
     the <literal>ORDER BY</literal> clause; <literal>ORDER BY</literal>
-    is mandatory in this case.
+    is mandatory in this case, and <literal>SKIP LOCKED</literal> is
+    not allowed.
     <literal>ROW</literal> and <literal>ROWS</literal> as well as
     <literal>FIRST</literal> and <literal>NEXT</literal> are noise
     words that don't influence the effects of these clauses.
diff --git a/src/backend/commands/matview.c b/src/backend/commands/matview.c
index 512b00bc65..fbbf769a87 100644
--- a/src/backend/commands/matview.c
+++ b/src/backend/commands/matview.c
@@ -185,7 +185,8 @@ ExecRefreshMatView(RefreshMatViewStmt *stmt, const char *queryString,
 	if (concurrent && stmt->skipData)
 		ereport(ERROR,
 				(errcode(ERRCODE_SYNTAX_ERROR),
-				 errmsg("CONCURRENTLY and WITH NO DATA options cannot be used together")));
+				 errmsg("%s and %s options cannot be used together",
+						"CONCURRENTLY", "WITH NO DATA")));
 
 	/*
 	 * Check that everything is correct for a refresh. Problems at this point
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index e3068a374e..08f1bf1031 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -16816,6 +16816,21 @@ insertSelectOptions(SelectStmt *stmt,
 			ereport(ERROR,
 					(errcode(ERRCODE_SYNTAX_ERROR),
 					 errmsg("WITH TIES cannot be specified without ORDER BY clause")));
+		if (limitClause->limitOption == LIMIT_OPTION_WITH_TIES && stmt->lockingClause)
+		{
+			ListCell   *lc;
+
+			foreach(lc, stmt->lockingClause)
+			{
+				LockingClause *lock = lfirst_node(LockingClause, lc);
+
+				if (lock->waitPolicy == LockWaitSkip)
+					ereport(ERROR,
+							(errcode(ERRCODE_SYNTAX_ERROR),
+							 errmsg("%s and %s options cannot be used together",
+									"SKIP LOCKED", "WITH TIES")));
+			}
+		}
 		stmt->limitOption = limitClause->limitOption;
 	}
 	if (withClause)
diff --git a/src/test/regress/expected/limit.out b/src/test/regress/expected/limit.out
index b75afcc01a..8a98bbea8e 100644
--- a/src/test/regress/expected/limit.out
+++ b/src/test/regress/expected/limit.out
@@ -619,6 +619,11 @@ SELECT  thousand
         0
 (2 rows)
 
+-- SKIP LOCKED and WITH TIES are incompatible
+SELECT  thousand
+		FROM onek WHERE thousand < 5
+		ORDER BY thousand FETCH FIRST 1 ROW WITH TIES FOR UPDATE SKIP LOCKED;
+ERROR:  SKIP LOCKED and WITH TIES options cannot be used together
 -- should fail
 SELECT ''::text AS two, unique1, unique2, stringu1
 		FROM onek WHERE unique1 > 50
diff --git a/src/test/regress/sql/limit.sql b/src/test/regress/sql/limit.sql
index d2d4ef132d..6f0cda9870 100644
--- a/src/test/regress/sql/limit.sql
+++ b/src/test/regress/sql/limit.sql
@@ -173,6 +173,11 @@ SELECT  thousand
 		FROM onek WHERE thousand < 5
 		ORDER BY thousand FETCH FIRST 2 ROW ONLY;
 
+-- SKIP LOCKED and WITH TIES are incompatible
+SELECT  thousand
+		FROM onek WHERE thousand < 5
+		ORDER BY thousand FETCH FIRST 1 ROW WITH TIES FOR UPDATE SKIP LOCKED;
+
 -- should fail
 SELECT ''::text AS two, unique1, unique2, stringu1
 		FROM onek WHERE unique1 > 50