Reject MERGE in CTEs and COPY

The grammar added for MERGE inadvertently made it accepted syntax in
places that were not prepared to deal with it -- namely COPY and inside
CTEs, but invoking these things with MERGE currently causes assertion
failures or weird misbehavior in non-assertion builds.  Protect those
places by checking for it explicitly until somebody decides to implement
it.

Reported-by: Alexey Borzov <borz_off@cs.msu.su>
Discussion: https://postgr.es/m/17579-82482cd7b267b862@postgresql.org

src/backend/commands/copy.c

@@ -273,6 +273,12 @@ DoCopy(ParseState *pstate, const CopyStmt *stmt,
 	{
 		Assert(stmt->query);
 
+		/* MERGE is allowed by parser, but unimplemented. Reject for now */
+		if (IsA(stmt->query, MergeStmt))
+			ereport(ERROR,
+					errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					errmsg("MERGE not supported in COPY"));
+
 		query = makeNode(RawStmt);
 		query->stmt = stmt->query;
 		query->stmt_location = stmt_location;

src/backend/parser/parse_cte.c

@@ -126,6 +126,13 @@ transformWithClause(ParseState *pstate, WithClause *withClause)
 		CommonTableExpr *cte = (CommonTableExpr *) lfirst(lc);
 		ListCell   *rest;
 
+		/* MERGE is allowed by parser, but unimplemented. Reject for now */
+		if (IsA(cte->ctequery, MergeStmt))
+			ereport(ERROR,
+					errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					errmsg("MERGE not supported in WITH query"),
+					parser_errposition(pstate, cte->location));
+
 		for_each_cell(rest, withClause->ctes, lnext(withClause->ctes, lc))
 		{
 			CommonTableExpr *cte2 = (CommonTableExpr *) lfirst(rest);

src/test/regress/expected/merge.out

@@ -123,6 +123,20 @@ ON tid = tid
 WHEN MATCHED THEN DO NOTHING;
 ERROR:  name "target" specified more than once
 DETAIL:  The name is used both as MERGE target table and data source.
+-- used in a CTE
+WITH foo AS (
+  MERGE INTO target USING source ON (true)
+  WHEN MATCHED THEN DELETE
+) SELECT * FROM foo;
+ERROR:  MERGE not supported in WITH query
+LINE 1: WITH foo AS (
+             ^
+-- used in COPY
+COPY (
+  MERGE INTO target USING source ON (true)
+  WHEN MATCHED THEN DELETE
+) TO stdout;
+ERROR:  MERGE not supported in COPY
 -- unsupported relation types
 -- view
 CREATE VIEW tv AS SELECT * FROM target;

src/test/regress/sql/merge.sql

@@ -88,6 +88,16 @@ MERGE INTO target
 USING target
 ON tid = tid
 WHEN MATCHED THEN DO NOTHING;
+-- used in a CTE
+WITH foo AS (
+  MERGE INTO target USING source ON (true)
+  WHEN MATCHED THEN DELETE
+) SELECT * FROM foo;
+-- used in COPY
+COPY (
+  MERGE INTO target USING source ON (true)
+  WHEN MATCHED THEN DELETE
+) TO stdout;
 
 -- unsupported relation types
 -- view