On Fri, 7 Jul 2023 17:21:36 +0900
Yugo NAGATA <[email protected]> wrote:
> Hi Nikita,
>
> On Wed, 5 Jul 2023 17:49:20 +0300
> Nikita Malakhov <[email protected]> wrote:
>
> > Hi!
> >
> > I like the idea of having a standard function which shows a TOAST value ID
> > for a row. I've used my own to handle TOAST errors. Just, maybe, more
> > correct
> > name would be "...value_id", because you actually retrieve valueid field
> > from the TOAST pointer, and chunk ID consists of valueid + chunk_seq.
>
> Thank you for your review!
>
> Although, the retrieved field is "va_valueid" and it is called "value ID" in
> the
> code, I chose the name "..._chunk_id" because I found the description in the
> documentation as followings:
>
> -------------
> Every TOAST table has the columns chunk_id (an OID identifying the particular
> TOASTed value), chunk_seq (a sequence number for the chunk within its value),
> and chunk_data (the actual data of the chunk). A unique index on chunk_id and
> chunk_seq provides fast retrieval of the values. A pointer datum representing
> an out-of-line on-disk TOASTed value therefore needs to store the OID of the
> TOAST table in which to look and the OID of the specific value (its chunk_id)
> -------------
> https://www.postgresql.org/docs/devel/storage-toast.html
>
> Here, chunk_id defined separately from chunk_seq. Therefore, I wonder
> pg_column_toast_chunk_id would be ok. However, I don't insist on this
> and I would be happy to change it if the other name is more natural for users.
I attached v2 patch that contains the documentation fix.
Regards,
Yugo Nagata
>
> Regards,
> Yugo Nagata
>
> >
> > --
> > Regards,
> > Nikita Malakhov
> > Postgres Professional
> > The Russian Postgres Company
> > https://postgrespro.ru/
>
>
> --
> Yugo NAGATA <[email protected]>
>
>
--
Yugo NAGATA <[email protected]>
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 5a47ce4343..c2c3156cd4 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -27608,6 +27608,20 @@ 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>
+ Returns the chunk id of the TOASTed value, or <literal>NULL</literal>
+ if the value is not TOASTed.
+ </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 884bfbc8ce..fe8788c1b1 100644
--- a/src/backend/utils/adt/varlena.c
+++ b/src/backend/utils/adt/varlena.c
@@ -5069,6 +5069,47 @@ pg_column_compression(PG_FUNCTION_ARGS)
PG_RETURN_TEXT_P(cstring_to_text(result));
}
+/*
+ * Return the chunk id of the TOASTed value.
+ * Return NULL for unTOASTed value.
+ */
+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 6996073989..0cacd0391d 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -7403,6 +7403,9 @@
{ 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 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',
