sql: check replace view columns earlier

Before this change, we could encounter internal errors while attempting
to add result columns during a `CREATE OR REPLACE VIEW` if the number of
columns in the new view was less than the number of columns in the old
view. This led to an inconsistency with postgres, which would only
return the error `cannot drop columns from view`.

This PR moves the check comparing the number of columns before and after
the view replacement earlier so that the correct error returns.

Co-authored-by: [email protected]

Fixes: #99000
Epic: None

Release note (bug fix): Fixes an internal error that can occur when
`CREATE OR REPLACE VIEW` replaces a view with fewer columns and another
entity depended on the view.

pkg/sql/create_view.go

@@ -596,6 +596,11 @@ func (p *planner) replaceViewDesc(
 		toReplace.ViewQuery = updatedQuery
 	}
 
+	// Check that the new view has at least as many columns as the old view before
+	// adding result columns.
+	if len(n.columns) < len(toReplace.ClusterVersion().Columns) {
+		return nil, pgerror.Newf(pgcode.InvalidTableDefinition, "cannot drop columns from view")
+	}
 	// Reset the columns to add the new result columns onto.
 	toReplace.Columns = make([]descpb.ColumnDescriptor, 0, len(n.columns))
 	toReplace.NextColumnID = 0

pkg/sql/logictest/testdata/logic_test/views

@@ -1813,3 +1813,25 @@ SELECT * FROM v;
 
 statement ok
 SET DATABASE = test;
+
+# When replacing a view with fewer columns, an error about dropping columns from
+# views preempts any errors about dependencies.
+subtest regression_99000
+
+statement ok
+CREATE TABLE films (id int PRIMARY KEY, title text, kind text, classification CHAR(1));
+
+statement ok
+CREATE VIEW comedies AS
+    SELECT *
+    FROM films
+    WHERE kind = 'Comedy';
+
+statement ok
+CREATE VIEW pg_comedies AS
+    SELECT *
+    FROM comedies
+    WHERE classification = 'PG';
+
+statement error pq: cannot drop columns from view
+CREATE OR REPLACE VIEW comedies AS SELECT ARRAY[films.*]::string FROM films;