Improve error handling of column references in expression transformation

Column references are not allowed in default expressions and partition
bound expressions, and are restricted as such once the transformation of
their expressions is done.  However, trying to use more complex column
references can lead to confusing error messages.  For example, trying to
use a two-field column reference name for default expressions and
partition bounds leads to "missing FROM-clause entry for table", which
makes no sense in their respective context.

In order to make the errors generated more useful, this commit adds more
verbose messages when transforming column references depending on the
context.  This has a little consequence though: for example an
expression using an aggregate with a column reference as argument would
cause an error to be generated for the column reference, while the
aggregate was the problem reported before this commit because column
references get transformed first.

The confusion exists for default expressions for a long time, and the
problem is new as of v12 for partition bounds.  Still per the lack of
complaints on the matter no backpatch is done.

The patch has been written by Amit Langote and me, and Tom Lane has
provided the improvement of the documentation for default expressions on
the CREATE TABLE page.

Author: Amit Langote, Michael Paquier
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/20190326020853.GM2558@paquier.xyz

1 files changed, 73 insertions, 0 deletions

diff --git a/src/backend/parser/parse_expr.c b/src/backend/parser/parse_expr.c
index e5593535292..3e648dc8ef9 100644
--- a/src/backend/parser/parse_expr.c
+++ b/src/backend/parser/parse_expr.c
@@ -520,6 +520,79 @@ transformColumnRef(ParseState *pstate, ColumnRef *cref)
 		CRERR_WRONG_DB,
 		CRERR_TOO_MANY
 	}			crerr = CRERR_NO_COLUMN;
+	const char *err;
+
+	/*
+	 * Check to see if the column reference is in an invalid place within the
+	 * query.  We allow column references in most places, except in default
+	 * expressions and partition bound expressions.
+	 */
+	err = NULL;
+	switch (pstate->p_expr_kind)
+	{
+		case EXPR_KIND_NONE:
+			Assert(false);		/* can't happen */
+			break;
+		case EXPR_KIND_OTHER:
+		case EXPR_KIND_JOIN_ON:
+		case EXPR_KIND_JOIN_USING:
+		case EXPR_KIND_FROM_SUBSELECT:
+		case EXPR_KIND_FROM_FUNCTION:
+		case EXPR_KIND_WHERE:
+		case EXPR_KIND_POLICY:
+		case EXPR_KIND_HAVING:
+		case EXPR_KIND_FILTER:
+		case EXPR_KIND_WINDOW_PARTITION:
+		case EXPR_KIND_WINDOW_ORDER:
+		case EXPR_KIND_WINDOW_FRAME_RANGE:
+		case EXPR_KIND_WINDOW_FRAME_ROWS:
+		case EXPR_KIND_WINDOW_FRAME_GROUPS:
+		case EXPR_KIND_SELECT_TARGET:
+		case EXPR_KIND_INSERT_TARGET:
+		case EXPR_KIND_UPDATE_SOURCE:
+		case EXPR_KIND_UPDATE_TARGET:
+		case EXPR_KIND_GROUP_BY:
+		case EXPR_KIND_ORDER_BY:
+		case EXPR_KIND_DISTINCT_ON:
+		case EXPR_KIND_LIMIT:
+		case EXPR_KIND_OFFSET:
+		case EXPR_KIND_RETURNING:
+		case EXPR_KIND_VALUES:
+		case EXPR_KIND_VALUES_SINGLE:
+		case EXPR_KIND_CHECK_CONSTRAINT:
+		case EXPR_KIND_DOMAIN_CHECK:
+		case EXPR_KIND_FUNCTION_DEFAULT:
+		case EXPR_KIND_INDEX_EXPRESSION:
+		case EXPR_KIND_INDEX_PREDICATE:
+		case EXPR_KIND_ALTER_COL_TRANSFORM:
+		case EXPR_KIND_EXECUTE_PARAMETER:
+		case EXPR_KIND_TRIGGER_WHEN:
+		case EXPR_KIND_PARTITION_EXPRESSION:
+		case EXPR_KIND_CALL_ARGUMENT:
+		case EXPR_KIND_COPY_WHERE:
+			/* okay */
+			break;
+
+		case EXPR_KIND_COLUMN_DEFAULT:
+			err = _("cannot use column reference in DEFAULT expression");
+			break;
+		case EXPR_KIND_PARTITION_BOUND:
+			err = _("cannot use column reference in partition bound expression");
+			break;
+
+			/*
+			 * There is intentionally no default: case here, so that the
+			 * compiler will warn if we add a new ParseExprKind without
+			 * extending this switch.  If we do see an unrecognized value at
+			 * runtime, the behavior will be the same as for EXPR_KIND_OTHER,
+			 * which is sane anyway.
+			 */
+	}
+	if (err)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg_internal("%s", err),
+				 parser_errposition(pstate, cref->location)));
 
 	/*
 	 * Give the PreParseColumnRefHook, if any, first shot.  If it returns