Disallow set-returning functions inside CASE or COALESCE.

When we reimplemented SRFs in commit 69f4b9c85, our initial choice was
to allow the behavior to vary from historical practice in cases where a
SRF call appeared within a conditional-execution construct (currently,
only CASE or COALESCE).  But that was controversial to begin with, and
subsequent discussion has resulted in a consensus that it's better to
throw an error instead of executing the query differently from before,
so long as we can provide a reasonably clear error message and a way to
rewrite the query.

Hence, add a parser mechanism to allow detection of such cases during
parse analysis.  The mechanism just requires storing, in the ParseState,
a pointer to the set-returning FuncExpr or OpExpr most recently emitted
by parse analysis.  Then the parsing functions for CASE and COALESCE can
detect the presence of a SRF in their arguments by noting whether this
pointer changes while analyzing their arguments.  Furthermore, if it does,
it provides a suitable error cursor location for the complaint.  (This
means that if there's more than one SRF in the arguments, the error will
point at the last one to be analyzed not the first.  While connoisseurs of
parsing behavior might find that odd, it's unlikely the average user would
ever notice.)

While at it, we can also provide more specific error messages than before
about some pre-existing restrictions, such as no-SRFs-within-aggregates.
Also, reject at parse time cases where a NULLIF or IS DISTINCT FROM
construct would need to return a set.  We've never supported that, but the
restriction is depended on in more subtle ways now, so it seems wise to
detect it at the start.

Also, provide some documentation about how to rewrite a SRF-within-CASE
query using a custom wrapper SRF.

It turns out that the information_schema.user_mapping_options view
contained an instance of exactly the behavior we're now forbidding; but
rewriting it makes it more clear and safer too.

initdb forced because of user_mapping_options change.

Patch by me, with error message suggestions from Alvaro Herrera and
Andres Freund, pursuant to a complaint from Regina Obe.

Discussion: https://postgr.es/m/000001d2d5de$d8d66170$8a832450$@pcorp.us

1 files changed, 36 insertions, 5 deletions

diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 55853c20bb4..34f1cf82ee0 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -64,10 +64,14 @@ static Node *ParseComplexProjection(ParseState *pstate, char *funcname,
  *
  *	The argument expressions (in fargs) must have been transformed
  *	already.  However, nothing in *fn has been transformed.
+ *
+ *	last_srf should be a copy of pstate->p_last_srf from just before we
+ *	started transforming fargs.  If the caller knows that fargs couldn't
+ *	contain any SRF calls, last_srf can just be pstate->p_last_srf.
  */
 Node *
 ParseFuncOrColumn(ParseState *pstate, List *funcname, List *fargs,
-				  FuncCall *fn, int location)
+				  Node *last_srf, FuncCall *fn, int location)
 {
 	bool		is_column = (fn == NULL);
 	List	   *agg_order = (fn ? fn->agg_order : NIL);
@@ -628,7 +632,7 @@ ParseFuncOrColumn(ParseState *pstate, List *funcname, List *fargs,
 
 	/* if it returns a set, check that's OK */
 	if (retset)
-		check_srf_call_placement(pstate, location);
+		check_srf_call_placement(pstate, last_srf, location);
 
 	/* build the appropriate output structure */
 	if (fdresult == FUNCDETAIL_NORMAL)
@@ -759,6 +763,17 @@ ParseFuncOrColumn(ParseState *pstate, List *funcname, List *fargs,
 					 errmsg("FILTER is not implemented for non-aggregate window functions"),
 					 parser_errposition(pstate, location)));
 
+		/*
+		 * Window functions can't either take or return sets
+		 */
+		if (pstate->p_last_srf != last_srf)
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("window function calls cannot contain set-returning function calls"),
+					 errhint("You might be able to move the set-returning function into a LATERAL FROM item."),
+					 parser_errposition(pstate,
+										exprLocation(pstate->p_last_srf))));
+
 		if (retset)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_FUNCTION_DEFINITION),
@@ -771,6 +786,10 @@ ParseFuncOrColumn(ParseState *pstate, List *funcname, List *fargs,
 		retval = (Node *) wfunc;
 	}
 
+	/* if it returns a set, remember it for error checks at higher levels */
+	if (retset)
+		pstate->p_last_srf = retval;
+
 	return retval;
 }
 
@@ -2083,9 +2102,13 @@ LookupAggWithArgs(ObjectWithArgs *agg, bool noError)
  *		and throw a nice error if not.
  *
  * A side-effect is to set pstate->p_hasTargetSRFs true if appropriate.
+ *
+ * last_srf should be a copy of pstate->p_last_srf from just before we
+ * started transforming the function's arguments.  This allows detection
+ * of whether the SRF's arguments contain any SRFs.
  */
 void
-check_srf_call_placement(ParseState *pstate, int location)
+check_srf_call_placement(ParseState *pstate, Node *last_srf, int location)
 {
 	const char *err;
 	bool		errkind;
@@ -2121,7 +2144,15 @@ check_srf_call_placement(ParseState *pstate, int location)
 			errkind = true;
 			break;
 		case EXPR_KIND_FROM_FUNCTION:
-			/* okay ... but we can't check nesting here */
+			/* okay, but we don't allow nested SRFs here */
+			/* errmsg is chosen to match transformRangeFunction() */
+			/* errposition should point to the inner SRF */
+			if (pstate->p_last_srf != last_srf)
+				ereport(ERROR,
+						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+						 errmsg("set-returning functions must appear at top level of FROM"),
+						 parser_errposition(pstate,
+										 exprLocation(pstate->p_last_srf))));
 			break;
 		case EXPR_KIND_WHERE:
 			errkind = true;
@@ -2202,7 +2233,7 @@ check_srf_call_placement(ParseState *pstate, int location)
 			err = _("set-returning functions are not allowed in trigger WHEN conditions");
 			break;
 		case EXPR_KIND_PARTITION_EXPRESSION:
-			err = _("set-returning functions are not allowed in partition key expression");
+			err = _("set-returning functions are not allowed in partition key expressions");
 			break;
 
 			/*