Remove "parent" column from pg_backend_memory_contexts

32d3ed816 added the "path" column to pg_backend_memory_contexts to allow
a stable method of obtaining the parent MemoryContext of a given row in
the view.  Using the "path" column is now the preferred method of
obtaining the parent row.

Previously, any queries which were self-joining to this view using the
"name" and "parent" columns could get incorrect results due to the fact
that names are not unique.  Here we aim to explicitly break such queries
so that they can be corrected and use the "path" column instead.

It is possible that there are more innocent users of the parent column
that just need an indication of the parent and having to write out a
self-joining CTE may be an unnecessary hassle for those cases.  Let's
remove the column for now and see if anyone comes back with any
complaints.  This does seem like a good time to attempt to get rid of
the column as we still have around 1 year to revert this if someone comes
back with a valid complaint.  Plus this view is new to v14 and is quite
niche, so perhaps not many people will be affected.

Author: Melih Mutlu <m.melihmutlu@gmail.com>
Discussion: https://postgr.es/m/CAGPVpCT7NOe4fZXRL8XaoxHpSXYTu6GTpULT_3E-HT9hzjoFRA@mail.gmail.com

doc/src/sgml/system-views.sgml

@@ -481,15 +481,6 @@
       </para></entry>
      </row>
 
-     <row>
-      <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>parent</structfield> <type>text</type>
-      </para>
-      <para>
-       Name of the parent of this memory context
-      </para></entry>
-     </row>
-
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>type</structfield> <type>text</type>

src/backend/utils/adt/mcxtfuncs.c

@@ -40,28 +40,6 @@ typedef struct MemoryContextId
 	int			context_id;
 }			MemoryContextId;
 
-/*
- * get_memory_context_name_and_ident
- *		Populate *name and *ident from the name and ident from 'context'.
- */
-static void
-get_memory_context_name_and_ident(MemoryContext context, const char **const name,
-								  const char **const ident)
-{
-	*name = context->name;
-	*ident = context->ident;
-
-	/*
-	 * To be consistent with logging output, we label dynahash contexts with
-	 * just the hash table name as with MemoryContextStatsPrint().
-	 */
-	if (*ident == NULL && strcmp(*name, "dynahash") == 0)
-	{
-		*name = *ident;
-		*ident = NULL;
-	}
-}
-
 /*
  * int_list_to_array
  *		Convert an IntList to an array of INT4OIDs.
@@ -93,7 +71,7 @@ PutMemoryContextsStatsTupleStore(Tuplestorestate *tupstore,
 								 TupleDesc tupdesc, MemoryContext context,
 								 HTAB *context_id_lookup)
 {
-#define PG_GET_BACKEND_MEMORY_CONTEXTS_COLS	11
+#define PG_GET_BACKEND_MEMORY_CONTEXTS_COLS	10
 
 	Datum		values[PG_GET_BACKEND_MEMORY_CONTEXTS_COLS];
 	bool		nulls[PG_GET_BACKEND_MEMORY_CONTEXTS_COLS];
@@ -128,7 +106,18 @@ PutMemoryContextsStatsTupleStore(Tuplestorestate *tupstore,
 	memset(values, 0, sizeof(values));
 	memset(nulls, 0, sizeof(nulls));
 
-	get_memory_context_name_and_ident(context, &name, &ident);
+	name = context->name;
+	ident = context->ident;
+
+	/*
+	 * To be consistent with logging output, we label dynahash contexts with
+	 * just the hash table name as with MemoryContextStatsPrint().
+	 */
+	if (ident && strcmp(name, "dynahash") == 0)
+	{
+		name = ident;
+		ident = NULL;
+	}
 
 	if (name)
 		values[0] = CStringGetTextDatum(name);
@@ -154,18 +143,6 @@ PutMemoryContextsStatsTupleStore(Tuplestorestate *tupstore,
 	else
 		nulls[1] = true;
 
-	if (context->parent)
-	{
-		const char *parent_name,
-				   *parent_ident;
-
-		get_memory_context_name_and_ident(context->parent, &parent_name,
-										  &parent_ident);
-		values[2] = CStringGetTextDatum(parent_name);
-	}
-	else
-		nulls[2] = true;
-
 	switch (context->type)
 	{
 		case T_AllocSetContext:
@@ -185,14 +162,14 @@ PutMemoryContextsStatsTupleStore(Tuplestorestate *tupstore,
 			break;
 	}
 
-	values[3] = CStringGetTextDatum(type);
-	values[4] = Int32GetDatum(list_length(path));	/* level */
-	values[5] = int_list_to_array(path);
-	values[6] = Int64GetDatum(stat.totalspace);
-	values[7] = Int64GetDatum(stat.nblocks);
-	values[8] = Int64GetDatum(stat.freespace);
-	values[9] = Int64GetDatum(stat.freechunks);
-	values[10] = Int64GetDatum(stat.totalspace - stat.freespace);
+	values[2] = CStringGetTextDatum(type);
+	values[3] = Int32GetDatum(list_length(path));	/* level */
+	values[4] = int_list_to_array(path);
+	values[5] = Int64GetDatum(stat.totalspace);
+	values[6] = Int64GetDatum(stat.nblocks);
+	values[7] = Int64GetDatum(stat.freespace);
+	values[8] = Int64GetDatum(stat.freechunks);
+	values[9] = Int64GetDatum(stat.totalspace - stat.freespace);
 
 	tuplestore_putvalues(tupstore, tupdesc, values, nulls);
 	list_free(path);

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202408021
+#define CATALOG_VERSION_NO	202408121
 
 #endif

src/include/catalog/pg_proc.dat

@@ -8329,9 +8329,9 @@
   proname => 'pg_get_backend_memory_contexts', prorows => '100',
   proretset => 't', provolatile => 'v', proparallel => 'r',
   prorettype => 'record', proargtypes => '',
-  proallargtypes => '{text,text,text,text,int4,_int4,int8,int8,int8,int8,int8}',
-  proargmodes => '{o,o,o,o,o,o,o,o,o,o,o}',
-  proargnames => '{name, ident, parent, type, level, path, total_bytes, total_nblocks, free_bytes, free_chunks, used_bytes}',
+  proallargtypes => '{text,text,text,int4,_int4,int8,int8,int8,int8,int8}',
+  proargmodes => '{o,o,o,o,o,o,o,o,o,o}',
+  proargnames => '{name, ident, type, level, path, total_bytes, total_nblocks, free_bytes, free_chunks, used_bytes}',
   prosrc => 'pg_get_backend_memory_contexts' },
 
 # logging memory contexts of the specified backend

src/test/regress/expected/rules.out

@@ -1305,7 +1305,6 @@ pg_available_extensions| SELECT e.name,
      LEFT JOIN pg_extension x ON ((e.name = x.extname)));
 pg_backend_memory_contexts| SELECT name,
     ident,
-    parent,
     type,
     level,
     path,
@@ -1314,7 +1313,7 @@ pg_backend_memory_contexts| SELECT name,
     free_bytes,
     free_chunks,
     used_bytes
-   FROM pg_get_backend_memory_contexts() pg_get_backend_memory_contexts(name, ident, parent, type, level, path, total_bytes, total_nblocks, free_bytes, free_chunks, used_bytes);
+   FROM pg_get_backend_memory_contexts() pg_get_backend_memory_contexts(name, ident, type, level, path, total_bytes, total_nblocks, free_bytes, free_chunks, used_bytes);
 pg_config| SELECT name,
     setting
    FROM pg_config() pg_config(name, setting);

src/test/regress/expected/sysviews.out

@@ -21,11 +21,11 @@ select count(*) >= 0 as ok from pg_available_extensions;
 
 -- The entire output of pg_backend_memory_contexts is not stable,
 -- we test only the existence and basic condition of TopMemoryContext.
-select type, name, ident, parent, level, total_bytes >= free_bytes
+select type, name, ident, level, total_bytes >= free_bytes
   from pg_backend_memory_contexts where level = 1;
-   type   |       name       | ident | parent | level | ?column? 
-----------+------------------+-------+--------+-------+----------
- AllocSet | TopMemoryContext |       |        |     1 | t
+   type   |       name       | ident | level | ?column? 
+----------+------------------+-------+-------+----------
+ AllocSet | TopMemoryContext |       |     1 | t
 (1 row)
 
 -- We can exercise some MemoryContext type stats functions.  Most of the
@@ -43,11 +43,11 @@ fetch 1 from cur;
  bbbbbbbbbb | 2
 (1 row)
 
-select type, name, parent, total_bytes > 0, total_nblocks, free_bytes > 0, free_chunks
+select type, name, total_bytes > 0, total_nblocks, free_bytes > 0, free_chunks
 from pg_backend_memory_contexts where name = 'Caller tuples';
- type |     name      |     parent     | ?column? | total_nblocks | ?column? | free_chunks 
-------+---------------+----------------+----------+---------------+----------+-------------
- Bump | Caller tuples | TupleSort sort | t        |             2 | t        |           0
+ type |     name      | ?column? | total_nblocks | ?column? | free_chunks 
+------+---------------+----------+---------------+----------+-------------
+ Bump | Caller tuples | t        |             2 | t        |           0
 (1 row)
 
 rollback;

src/test/regress/sql/sysviews.sql

@@ -14,7 +14,7 @@ select count(*) >= 0 as ok from pg_available_extensions;
 
 -- The entire output of pg_backend_memory_contexts is not stable,
 -- we test only the existence and basic condition of TopMemoryContext.
-select type, name, ident, parent, level, total_bytes >= free_bytes
+select type, name, ident, level, total_bytes >= free_bytes
   from pg_backend_memory_contexts where level = 1;
 
 -- We can exercise some MemoryContext type stats functions.  Most of the
@@ -28,7 +28,7 @@ declare cur cursor for select left(a,10), b
   from (values(repeat('a', 512 * 1024),1),(repeat('b', 512),2)) v(a,b)
   order by v.a desc;
 fetch 1 from cur;
-select type, name, parent, total_bytes > 0, total_nblocks, free_bytes > 0, free_chunks
+select type, name, total_bytes > 0, total_nblocks, free_bytes > 0, free_chunks
 from pg_backend_memory_contexts where name = 'Caller tuples';
 rollback;