Add pg_column_toast_chunk_id().

This function returns the chunk_id of an on-disk TOASTed value.  If
the value is un-TOASTed or not on-disk, it returns NULL.  This is
useful for identifying which values are actually TOASTed and for
investigating "unexpected chunk number" errors.

Bumps catversion.

Author: Yugo Nagata
Reviewed-by: Jian He
Discussion: https://postgr.es/m/20230329105507.d764497456eeac1ca491b5bd%40sraoss.co.jp

doc/src/sgml/func.sgml

@@ -28552,6 +28552,23 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
        </para></entry>
       </row>
 
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_column_toast_chunk_id</primary>
+        </indexterm>
+        <function>pg_column_toast_chunk_id</function> ( <type>"any"</type> )
+        <returnvalue>oid</returnvalue>
+       </para>
+       <para>
+        Shows the <structfield>chunk_id</structfield> of an on-disk
+        <acronym>TOAST</acronym>ed value.  Returns <literal>NULL</literal>
+        if the value is un-<acronym>TOAST</acronym>ed or not on-disk.  See
+        <xref linkend="storage-toast"/> for more information about
+        <acronym>TOAST</acronym>.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>

src/backend/utils/adt/varlena.c

@@ -5105,6 +5105,47 @@ pg_column_compression(PG_FUNCTION_ARGS)
 	PG_RETURN_TEXT_P(cstring_to_text(result));
 }
 
+/*
+ * Return the chunk_id of the on-disk TOASTed value.  Return NULL if the value
+ * is un-TOASTed or not on-disk.
+ */
+Datum
+pg_column_toast_chunk_id(PG_FUNCTION_ARGS)
+{
+	int			typlen;
+	struct varlena *attr;
+	struct varatt_external toast_pointer;
+
+	/* On first call, get the input type's typlen, and save at *fn_extra */
+	if (fcinfo->flinfo->fn_extra == NULL)
+	{
+		/* Lookup the datatype of the supplied argument */
+		Oid			argtypeid = get_fn_expr_argtype(fcinfo->flinfo, 0);
+
+		typlen = get_typlen(argtypeid);
+		if (typlen == 0)		/* should not happen */
+			elog(ERROR, "cache lookup failed for type %u", argtypeid);
+
+		fcinfo->flinfo->fn_extra = MemoryContextAlloc(fcinfo->flinfo->fn_mcxt,
+													  sizeof(int));
+		*((int *) fcinfo->flinfo->fn_extra) = typlen;
+	}
+	else
+		typlen = *((int *) fcinfo->flinfo->fn_extra);
+
+	if (typlen != -1)
+		PG_RETURN_NULL();
+
+	attr = (struct varlena *) DatumGetPointer(PG_GETARG_DATUM(0));
+
+	if (!VARATT_IS_EXTERNAL_ONDISK(attr))
+		PG_RETURN_NULL();
+
+	VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
+
+	PG_RETURN_OID(toast_pointer.va_valueid);
+}
+
 /*
  * string_agg - Concatenates values and returns string.
  *

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202403132
+#define CATALOG_VERSION_NO	202403141
 
 #endif

src/include/catalog/pg_proc.dat

@@ -7447,6 +7447,9 @@
 { oid => '2121', descr => 'compression method for the compressed datum',
   proname => 'pg_column_compression', provolatile => 's', prorettype => 'text',
   proargtypes => 'any', prosrc => 'pg_column_compression' },
+{ oid => '8393', descr => 'chunk ID of on-disk TOASTed value',
+  proname => 'pg_column_toast_chunk_id', provolatile => 's', prorettype => 'oid',
+  proargtypes => 'any', prosrc => 'pg_column_toast_chunk_id' },
 { oid => '2322',
   descr => 'total disk space usage for the specified tablespace',
   proname => 'pg_tablespace_size', provolatile => 'v', prorettype => 'int8',

src/test/regress/expected/misc_functions.out

@@ -703,3 +703,19 @@ SELECT has_function_privilege('regress_current_logfile',
 (1 row)
 
 DROP ROLE regress_current_logfile;
+-- pg_column_toast_chunk_id
+CREATE TABLE test_chunk_id (a TEXT, b TEXT STORAGE EXTERNAL);
+INSERT INTO test_chunk_id VALUES ('x', repeat('x', 8192));
+SELECT t.relname AS toastrel FROM pg_class c
+  LEFT JOIN pg_class t ON c.reltoastrelid = t.oid
+  WHERE c.relname = 'test_chunk_id'
+\gset
+SELECT pg_column_toast_chunk_id(a) IS NULL,
+  pg_column_toast_chunk_id(b) IN (SELECT chunk_id FROM pg_toast.:toastrel)
+  FROM test_chunk_id;
+ ?column? | ?column? 
+----------+----------
+ t        | t
+(1 row)
+
+DROP TABLE test_chunk_id;

src/test/regress/sql/misc_functions.sql

@@ -265,3 +265,15 @@ GRANT pg_monitor TO regress_current_logfile;
 SELECT has_function_privilege('regress_current_logfile',
   'pg_current_logfile()', 'EXECUTE');
 DROP ROLE regress_current_logfile;
+
+-- pg_column_toast_chunk_id
+CREATE TABLE test_chunk_id (a TEXT, b TEXT STORAGE EXTERNAL);
+INSERT INTO test_chunk_id VALUES ('x', repeat('x', 8192));
+SELECT t.relname AS toastrel FROM pg_class c
+  LEFT JOIN pg_class t ON c.reltoastrelid = t.oid
+  WHERE c.relname = 'test_chunk_id'
+\gset
+SELECT pg_column_toast_chunk_id(a) IS NULL,
+  pg_column_toast_chunk_id(b) IN (SELECT chunk_id FROM pg_toast.:toastrel)
+  FROM test_chunk_id;
+DROP TABLE test_chunk_id;