Re: [GENERAL] When updating row that has TOAST column, is the TOASTcolumn also reinserted ? Or just the oid to the value? - Mailing list pgsql-general

Dorian Hoxha wrote:
> When updating row that has TOAST column, is the TOAST column also inserted ? Or just the oid?
> 
> Say I have a 1MB value in the TOAST column, and I update the row by changing another column, and since
> every update is an insert, will it also reinsert the toast-column ? The column that I will update will
> have an index so I think hot-update won't work in this case ? The same question also when full-page-
> writes is enabled ?
> 
> 
> Using 9.6.

The TOAST table will remain unchanged by the UPDATE; you can see that with the
"pageinspect" contrib module:

CREATE TABLE longtext (
   id integer primary key,
   val text NOT NULL,
   other integer NOT NULL
);

INSERT INTO longtext VALUES (
   4,
   (SELECT string_agg(chr((random()*25+65)::integer), '')
       FROM generate_series(1, 2000)),
   42
);

SELECT reltoastrelid, reltoastrelid::regclass FROM pg_class
   WHERE oid = 'longtext'::regclass;                              

 reltoastrelid |      reltoastrelid
---------------+-------------------------
         25206 | pg_toast.pg_toast_25203
(1 row)

Use "pageinspect" to see the tuples in the table and the TOAST table:

SELECT t_xmin, t_xmax, t_ctid, t_attrs[1] AS id, t_attrs[2] AS val, t_attrs[3] AS other
   FROM heap_page_item_attrs(get_raw_page('longtext', 0), 25203);

 t_xmin | t_xmax | t_ctid |     id     |                  val                   |   other
--------+--------+--------+------------+----------------------------------------+------------
   2076 |      0 | (0,1)  | \x04000000 | \x0112d4070000d00700007b62000076620000 | \x2a000000
(1 row)

SELECT t_xmin, t_xmax, t_ctid, t_attrs[1] AS chunk_id, t_attrs[2] AS chunk_seq
   FROM heap_page_item_attrs(get_raw_page('pg_toast.pg_toast_25203', 0), 25206);

 t_xmin | t_xmax | t_ctid |  chunk_id  | chunk_seq
--------+--------+--------+------------+------------
   2076 |      0 | (0,1)  | \x7b620000 | \x00000000
   2076 |      0 | (0,2)  | \x7b620000 | \x01000000
(2 rows)

Now let's UPDATE:

UPDATE longtext SET other = -1 WHERE id = 4;

Let's look at the tuples again:

SELECT t_xmin, t_xmax, t_ctid, t_attrs[1] AS id, t_attrs[2] AS val, t_attrs[3] AS other
   FROM heap_page_item_attrs(get_raw_page('longtext', 0), 25203);

 t_xmin | t_xmax | t_ctid |     id     |                  val                   |   other
--------+--------+--------+------------+----------------------------------------+------------
   2076 |   2077 | (0,2)  | \x04000000 | \x0112d4070000d00700007b62000076620000 | \x2a000000
   2077 |      0 | (0,2)  | \x04000000 | \x0112d4070000d00700007b62000076620000 | \xffffffff
(2 rows)

A new tuple has been entered, but "val" still points to chunk ID 0x0000627b
(this is a little-endian machine).

SELECT t_xmin, t_xmax, t_ctid, t_attrs[1] AS chunk_id, t_attrs[2] AS chunk_seq
   FROM heap_page_item_attrs(get_raw_page('pg_toast.pg_toast_25203', 0), 25206);

 t_xmin | t_xmax | t_ctid |  chunk_id  | chunk_seq
--------+--------+--------+------------+------------
   2076 |      0 | (0,1)  | \x7b620000 | \x00000000
   2076 |      0 | (0,2)  | \x7b620000 | \x01000000
(2 rows)

The TOAST table is unchanged!

This was a HOT update, but it works the same for a non-HOT update:

UPDATE longtext SET id = 1 WHERE id = 4;

SELECT t_xmin, t_xmax, t_ctid, t_attrs[1] AS id, t_attrs[2] AS val, t_attrs[3] AS other
   FROM heap_page_item_attrs(get_raw_page('longtext', 0), 25203);

 t_xmin | t_xmax | t_ctid |     id     |                  val                   |   other
--------+--------+--------+------------+----------------------------------------+------------
   2076 |   2077 | (0,2)  | \x04000000 | \x0112d4070000d00700007b62000076620000 | \x2a000000
   2077 |   2078 | (0,3)  | \x04000000 | \x0112d4070000d00700007b62000076620000 | \xffffffff
   2078 |      0 | (0,3)  | \x01000000 | \x0112d4070000d00700007b62000076620000 | \xffffffff
(3 rows)

SELECT t_xmin, t_xmax, t_ctid, t_attrs[1] AS chunk_id, t_attrs[2] AS chunk_seq
   FROM heap_page_item_attrs(get_raw_page('pg_toast.pg_toast_25203', 0), 25206);

 t_xmin | t_xmax | t_ctid |  chunk_id  | chunk_seq
--------+--------+--------+------------+------------
   2076 |      0 | (0,1)  | \x7b620000 | \x00000000
   2076 |      0 | (0,2)  | \x7b620000 | \x01000000
(2 rows)

Yours,
Laurenz Albe