Fix ruleutils issues with dropped cols in functions-returning-composite.

Due to lack of concern for the case in the dependency code, it's
possible to drop a column of a composite type even though stored
queries have references to the dropped column via functions-in-FROM
that return the composite type.  There are "soft" references,
namely FROM-clause aliases for such columns, and "hard" references,
that is actual Vars referring to them.  The right fix for hard
references is to add dependencies preventing the drop; something
we've known for many years and not done (and this commit still doesn't
address it).  A "soft" reference shouldn't prevent a drop though.
We've been around on this before (cf. 9b35ddce9, 2c4debbd0), but
nobody had noticed that the current behavior can result in dump/reload
failures, because ruleutils.c can print more column aliases than the
underlying composite type now has.  So we need to rejigger the
column-alias-handling code to treat such columns as dropped and not
print aliases for them.

Rather than writing new code for this, I used expandRTE() which already
knows how to figure out which function result columns are dropped.
I'd initially thought maybe we could use expandRTE() in all cases, but
that fails for EXPLAIN's purposes, because the planner strips a lot of
RTE infrastructure that expandRTE() needs.  So this patch just uses it
for unplanned function RTEs and otherwise does things the old way.

If there is a hard reference (Var), then removing the column alias
causes us to fail to print the Var, since there's no longer a name
to print.  Failing seems less desirable than printing a made-up
name, so I made it print "?dropped?column?" instead.

Per report from Timo Stolz.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/5c91267e-3b6d-5795-189c-d15a55d61dbb@nullachtvierzehn.de

src/backend/parser/parse_relation.c

@@ -3164,6 +3164,9 @@ expandNSItemAttrs(ParseState *pstate, ParseNamespaceItem *nsitem,
  *
  * "*" is returned if the given attnum is InvalidAttrNumber --- this case
  * occurs when a Var represents a whole tuple of a relation.
+ *
+ * It is caller's responsibility to not call this on a dropped attribute.
+ * (You will get some answer for such cases, but it might not be sensible.)
  */
 char *
 get_rte_attribute_name(RangeTblEntry *rte, AttrNumber attnum)

src/backend/utils/adt/ruleutils.c

@@ -53,6 +53,7 @@
 #include "parser/parse_func.h"
 #include "parser/parse_node.h"
 #include "parser/parse_oper.h"
+#include "parser/parse_relation.h"
 #include "parser/parser.h"
 #include "parser/parsetree.h"
 #include "rewrite/rewriteHandler.h"
@@ -4222,9 +4223,9 @@ set_relation_column_names(deparse_namespace *dpns, RangeTblEntry *rte,
 	int			j;
 
 	/*
-	 * Extract the RTE's "real" column names.  This is comparable to
-	 * get_rte_attribute_name, except that it's important to disregard dropped
-	 * columns.  We put NULL into the array for a dropped column.
+	 * Construct an array of the current "real" column names of the RTE.
+	 * real_colnames[] will be indexed by physical column number, with NULL
+	 * entries for dropped columns.
 	 */
 	if (rte->rtekind == RTE_RELATION)
 	{
@@ -4251,19 +4252,43 @@ set_relation_column_names(deparse_namespace *dpns, RangeTblEntry *rte,
 	}
 	else
 	{
-		/* Otherwise use the column names from eref */
+		/* Otherwise get the column names from eref or expandRTE() */
+		List	   *colnames;
 		ListCell   *lc;
 
-		ncolumns = list_length(rte->eref->colnames);
+		/*
+		 * Functions returning composites have the annoying property that some
+		 * of the composite type's columns might have been dropped since the
+		 * query was parsed.  If possible, use expandRTE() to handle that
+		 * case, since it has the tedious logic needed to find out about
+		 * dropped columns.  However, if we're explaining a plan, then we
+		 * don't have rte->functions because the planner thinks that won't be
+		 * needed later, and that breaks expandRTE().  So in that case we have
+		 * to rely on rte->eref, which may lead us to report a dropped
+		 * column's old name; that seems close enough for EXPLAIN's purposes.
+		 *
+		 * For non-RELATION, non-FUNCTION RTEs, we can just look at rte->eref,
+		 * which should be sufficiently up-to-date: no other RTE types can
+		 * have columns get dropped from under them after parsing.
+		 */
+		if (rte->rtekind == RTE_FUNCTION && rte->functions != NIL)
+		{
+			/* Since we're not creating Vars, rtindex etc. don't matter */
+			expandRTE(rte, 1, 0, -1, true /* include dropped */ ,
+					  &colnames, NULL);
+		}
+		else
+			colnames = rte->eref->colnames;
+
+		ncolumns = list_length(colnames);
 		real_colnames = (char **) palloc(ncolumns * sizeof(char *));
 
 		i = 0;
-		foreach(lc, rte->eref->colnames)
+		foreach(lc, colnames)
 		{
 			/*
-			 * If the column name shown in eref is an empty string, then it's
-			 * a column that was dropped at the time of parsing the query, so
-			 * treat it as dropped.
+			 * If the column name we find here is an empty string, then it's a
+			 * dropped column, so change to NULL.
 			 */
 			char	   *cname = strVal(lfirst(lc));
 
@@ -7192,9 +7217,16 @@ get_variable(Var *var, int levelsup, bool istoplevel, deparse_context *context)
 			elog(ERROR, "invalid attnum %d for relation \"%s\"",
 				 attnum, rte->eref->aliasname);
 		attname = colinfo->colnames[attnum - 1];
-		if (attname == NULL)	/* dropped column? */
-			elog(ERROR, "invalid attnum %d for relation \"%s\"",
-				 attnum, rte->eref->aliasname);
+
+		/*
+		 * If we find a Var referencing a dropped column, it seems better to
+		 * print something (anything) than to fail.  In general this should
+		 * not happen, but there are specific cases involving functions
+		 * returning named composite types where we don't sufficiently enforce
+		 * that you can't drop a column that's referenced in some view.
+		 */
+		if (attname == NULL)
+			attname = "?dropped?column?";
 	}
 	else
 	{

src/test/regress/expected/create_view.out

@@ -1546,17 +1546,26 @@ select * from tt14v;
 begin;
 -- this perhaps should be rejected, but it isn't:
 alter table tt14t drop column f3;
--- f3 is still in the view ...
+-- column f3 is still in the view, sort of ...
 select pg_get_viewdef('tt14v', true);
-         pg_get_viewdef         
---------------------------------
-  SELECT t.f1,                 +
-     t.f3,                     +
-     t.f4                      +
-    FROM tt14f() t(f1, f3, f4);
+         pg_get_viewdef          
+---------------------------------
+  SELECT t.f1,                  +
+     t."?dropped?column?" AS f3,+
+     t.f4                       +
+    FROM tt14f() t(f1, f4);
 (1 row)
 
--- but will fail at execution
+-- ... and you can even EXPLAIN it ...
+explain (verbose, costs off) select * from tt14v;
+               QUERY PLAN               
+----------------------------------------
+ Function Scan on testviewschm2.tt14f t
+   Output: t.f1, t.f3, t.f4
+   Function Call: tt14f()
+(3 rows)
+
+-- but it will fail at execution
 select f1, f4 from tt14v;
  f1  | f4 
 -----+----

src/test/regress/sql/create_view.sql

@@ -526,9 +526,11 @@ begin;
 -- this perhaps should be rejected, but it isn't:
 alter table tt14t drop column f3;
 
--- f3 is still in the view ...
+-- column f3 is still in the view, sort of ...
 select pg_get_viewdef('tt14v', true);
--- but will fail at execution
+-- ... and you can even EXPLAIN it ...
+explain (verbose, costs off) select * from tt14v;
+-- but it will fail at execution
 select f1, f4 from tt14v;
 select * from tt14v;