more comments

posthog/hogql/printer.py

@@ -236,8 +236,9 @@ def visit_select_query(self, node: ast.SelectQuery):
             next_join = next_join.next_join
 
         if node.select:
+            # Only for ClickHouse: Gather all visible aliases, and/or the last hidden alias for
+            # each unique alias name. Then make the last hidden aliases visible.
             if self.dialect == "clickhouse":
-                # Gather all visible aliases. Make visible the last occurrence of each unique hidden alias.
                 visible_aliases = {}
                 for alias in reversed(node.select):
                     if isinstance(alias, ast.Alias):
@@ -247,19 +248,27 @@ def visit_select_query(self, node: ast.SelectQuery):
                 columns = []
                 for column in node.select:
                     if isinstance(column, ast.Alias):
+                        # It's either a visible alias, or the last hidden alias for this name.
                         if visible_aliases.get(column.alias) == column:
                             if column.hidden:
                                 if (
                                     isinstance(column.expr, ast.Field)
                                     and isinstance(column.expr.type, ast.FieldType)
                                     and column.expr.type.name == column.alias
                                 ):
-                                    # Unhide if really the same name for the field and the alias
+                                    # Hide the hidden alias only if it's a simple field,
+                                    # and we're using the same name for the field and the alias
+                                    # E.g. events.event AS event --> events.evnet.
                                     column = column.expr
                                 else:
+                                    # Make the hidden alias visible
                                     column = cast(ast.Alias, clone_expr(column))
                                     column.hidden = False
+                            else:
+                                # Always print visible aliases.
+                                pass
                         else:
+                            # This is not the alias for this unique alias name. Skip it.
                             column = column.expr
                     columns.append(self.visit(column))
             else:
@@ -842,6 +851,7 @@ def visit_placeholder(self, node: ast.Placeholder):
         raise HogQLException(f"Placeholders, such as {{{node.field}}}, are not supported in this context")
 
     def visit_alias(self, node: ast.Alias):
+        # Skip hidden aliases completely.
         if node.hidden:
             return self.visit(node.expr)
         expr = node.expr

posthog/hogql/resolver.py

@@ -127,7 +127,7 @@ def visit_select_query(self, node: ast.SelectQuery):
             else:
                 select_nodes.append(new_expr)
 
-        columns_with_explicit_alias = {}
+        columns_with_visible_alias = {}
         for new_expr in select_nodes:
             if isinstance(new_expr.type, ast.FieldAliasType):
                 alias = new_expr.type.alias
@@ -139,14 +139,14 @@ def visit_select_query(self, node: ast.SelectQuery):
                 alias = None
 
             if alias:
-                # Remember the first visible or last hidden expr for each alias
+                # Make a reference of the first visible or last hidden expr for each unique alias name.
                 if isinstance(new_expr, ast.Alias) and new_expr.hidden:
-                    if alias not in node_type.columns or not columns_with_explicit_alias.get(alias, False):
+                    if alias not in node_type.columns or not columns_with_visible_alias.get(alias, False):
                         node_type.columns[alias] = new_expr.type
-                        columns_with_explicit_alias[alias] = False
+                        columns_with_visible_alias[alias] = False
                 else:
                     node_type.columns[alias] = new_expr.type
-                    columns_with_explicit_alias[alias] = True
+                    columns_with_visible_alias[alias] = True
 
             # add the column to the new select query
             new_node.select.append(new_expr)