Transform ALTER TABLE/SET TYPE/USING expr during parse analysis

This lets later stages have access to the transformed expression; in
particular it allows DDL-deparsing code during event triggers to pass
the transformed expression to ruleutils.c, so that the complete command
can be deparsed.

This shuffles the timing of the transform calls a bit: previously,
nothing was transformed during parse analysis, and only the
RELKIND_RELATION case was being handled during execution.  After this
patch, all expressions are transformed during parse analysis (including
those for relkinds other than RELATION), and the error for other
relation kinds is thrown only during execution.  So we do more work than
before to reject some bogus cases.  That seems acceptable.

src/backend/commands/tablecmds.c

@@ -7776,7 +7776,7 @@ ATPrepAlterColumnType(List **wqueue,
 	char	   *colName = cmd->name;
 	ColumnDef  *def = (ColumnDef *) cmd->def;
 	TypeName   *typeName = def->typeName;
-	Node	   *transform = def->raw_default;
+	Node	   *transform = def->cooked_default;
 	HeapTuple	tuple;
 	Form_pg_attribute attTup;
 	AttrNumber	attnum;
@@ -7835,34 +7835,13 @@ ATPrepAlterColumnType(List **wqueue,
 	{
 		/*
 		 * Set up an expression to transform the old data value to the new
-		 * type. If a USING option was given, transform and use that
-		 * expression, else just take the old value and try to coerce it.  We
-		 * do this first so that type incompatibility can be detected before
-		 * we waste effort, and because we need the expression to be parsed
-		 * against the original table row type.
+		 * type. If a USING option was given, use the expression as transformed
+		 * by transformAlterTableStmt, else just take the old value and try to
+		 * coerce it.  We do this first so that type incompatibility can be
+		 * detected before we waste effort, and because we need the expression
+		 * to be parsed against the original table row type.
 		 */
-		if (transform)
-		{
-			RangeTblEntry *rte;
-
-			/* Expression must be able to access vars of old table */
-			rte = addRangeTableEntryForRelation(pstate,
-												rel,
-												NULL,
-												false,
-												true);
-			addRTEtoQuery(pstate, rte, false, true, true);
-
-			transform = transformExpr(pstate, transform,
-									  EXPR_KIND_ALTER_COL_TRANSFORM);
-
-			/* It can't return a set */
-			if (expression_returns_set(transform))
-				ereport(ERROR,
-						(errcode(ERRCODE_DATATYPE_MISMATCH),
-					  errmsg("transform expression must not return a set")));
-		}
-		else
+		if (!transform)
 		{
 			transform = (Node *) makeVar(1, attnum,
 										 attTup->atttypid, attTup->atttypmod,

src/backend/parser/parse_utilcmd.c

@@ -2372,6 +2372,7 @@ transformAlterTableStmt(Oid relid, AlterTableStmt *stmt,
 	List	   *newcmds = NIL;
 	bool		skipValidation = true;
 	AlterTableCmd *newcmd;
+	RangeTblEntry *rte;
 
 	/*
 	 * We must not scribble on the passed-in AlterTableStmt, so copy it. (This
@@ -2382,10 +2383,17 @@ transformAlterTableStmt(Oid relid, AlterTableStmt *stmt,
 	/* Caller is responsible for locking the relation */
 	rel = relation_open(relid, NoLock);
 
-	/* Set up pstate and CreateStmtContext */
+	/* Set up pstate */
 	pstate = make_parsestate(NULL);
 	pstate->p_sourcetext = queryString;
+	rte = addRangeTableEntryForRelation(pstate,
+										rel,
+										NULL,
+										false,
+										true);
+	addRTEtoQuery(pstate, rte, false, true, true);
 
+	/* Set up CreateStmtContext */
 	cxt.pstate = pstate;
 	if (stmt->relkind == OBJECT_FOREIGN_TABLE)
 	{
@@ -2413,8 +2421,8 @@ transformAlterTableStmt(Oid relid, AlterTableStmt *stmt,
 
 	/*
 	 * The only subtypes that currently require parse transformation handling
-	 * are ADD COLUMN and ADD CONSTRAINT.  These largely re-use code from
-	 * CREATE TABLE.
+	 * are ADD COLUMN, ADD CONSTRAINT and SET DATA TYPE.  These largely re-use
+	 * code from CREATE TABLE.
 	 */
 	foreach(lcmd, stmt->cmds)
 	{
@@ -2446,6 +2454,7 @@ transformAlterTableStmt(Oid relid, AlterTableStmt *stmt,
 					newcmds = lappend(newcmds, cmd);
 					break;
 				}
+
 			case AT_AddConstraint:
 
 				/*
@@ -2472,6 +2481,31 @@ transformAlterTableStmt(Oid relid, AlterTableStmt *stmt,
 				newcmds = lappend(newcmds, cmd);
 				break;
 
+			case AT_AlterColumnType:
+				{
+					ColumnDef *def = (ColumnDef *) cmd->def;
+
+					/*
+					 * For ALTER COLUMN TYPE, transform the USING clause if
+					 * one was specified.
+					 */
+					if (def->raw_default)
+					{
+						def->cooked_default =
+							transformExpr(pstate, def->raw_default,
+										  EXPR_KIND_ALTER_COL_TRANSFORM);
+
+						/* it can't return a set */
+						if (expression_returns_set(def->cooked_default))
+							ereport(ERROR,
+									(errcode(ERRCODE_DATATYPE_MISMATCH),
+									 errmsg("transform expression must not return a set")));
+					}
+
+					newcmds = lappend(newcmds, cmd);
+					break;
+				}
+
 			default:
 				newcmds = lappend(newcmds, cmd);
 				break;