From 97af1b2300ecf07a34923da87a9d84e6aa963956 Mon Sep 17 00:00:00 2001
From: Yugo Nagata <nagata@sraoss.co.jp>
Date: Wed, 29 Mar 2023 09:59:25 +0900
Subject: [PATCH v3] Add pg_column_toast_chunk_id function

This function returns the chunk_id of an on-disk TOASTed value, or
NULL if the value is un-TOASTed or not on disk. This enables users to
know which columns are actually TOASTed. This function is also useful
to identify a problematic row when an error like
  "ERROR:  unexpected chunk number ... (expected ...) for toast value"
occurs.
---
 doc/src/sgml/func.sgml          | 17 ++++++++++++++
 src/backend/utils/adt/varlena.c | 41 +++++++++++++++++++++++++++++++++
 src/include/catalog/pg_proc.dat |  3 +++
 3 files changed, 61 insertions(+)

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 210c7c0b02..2d82331323 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -28078,6 +28078,23 @@ 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-ondisk"/> for details about
+        <acronym>TOAST</acronym>.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/utils/adt/varlena.c b/src/backend/utils/adt/varlena.c
index 543afb66e5..84d36781a4 100644
--- a/src/backend/utils/adt/varlena.c
+++ b/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 unTOASTed 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.
  *
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 58811a6530..1d4521ac1f 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -7454,6 +7454,9 @@
 { 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',
-- 
2.25.1