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Get toast chunk_id from the user table tuples or from the toast index thanks to pageinspect

Introduction

TOAST stands for “The Oversized-Attribute Storage Technique” and allows to broke up large fields value into multiple physical rows (see the PostgreSQL documentation for more details).

The goal of this post is to provide a way to retrieve toast’s information from the user table tuples or from the toast index without querying the toast directly.

We will be able to link the user table tuples to the toast pages by using user table tuples and toast index data (not querying the toast at all).

Build the playground

Create a table with a TOAST-able field:

toasted=# CREATE TABLE bdttab (id int,message text,age int);
CREATE TABLE

toasted=# \d+ bdttab
                                   Table "public.bdttab"
 Column  |  Type   | Collation | Nullable | Default | Storage  | Stats target | Description
---------+---------+-----------+----------+---------+----------+--------------+-------------
 id      | integer |           |          |         | plain    |              |
 message | text    |           |          |         | extended |              |
 age     | integer |           |          |         | plain    |              |
Access method: heap

Insert 10 rows:

toasted=# INSERT INTO bdttab
SELECT 1,(SELECT
string_agg(chr(floor(random() * 26)::int + 65), '')
FROM generate_series(1,10000)),6
FROM generate_series(1,10);
INSERT 0 10

check the toast and toast index names:

toasted=# select r.relname,t.relname as toast,i.relname as toast_index from pg_class r, pg_class i, pg_index d, pg_class t where r.relname = 'bdttab' and d.indrelid = r.reltoastrelid and i.oid = d.indexrelid and t.oid = r.reltoastrelid;
relname | toast | toast_index
---------+-----------------+-----------------------
bdttab | pg_toast_192881 | pg_toast_192881_index
(1 row)

Retrieve the chunk_id for each tuple directly from the user table

The chunk_id is part of the tuple’s data as explained in this slide (coming from this presentation):

https://bdrouvot.files.wordpress.com/2020/01/toasted_attributes.png?w=15... 150w, https://bdrouvot.files.wordpress.com/2020/01/toasted_attributes.png?w=30... 300w, https://bdrouvot.files.wordpress.com/2020/01/toasted_attributes.png?w=76... 768w, https://bdrouvot.files.wordpress.com/2020/01/toasted_attributes.png?w=10... 1024w, https://bdrouvot.files.wordpress.com/2020/01/toasted_attributes.png 2082w" sizes="(max-width: 1100px) 100vw, 1100px" />

Note that the toast relation id is also part of the tuple data.

As you can see from the slide we can get the information with pageinspect, so let’s use it to build a query to retrieve the chunk_id and the toast relation id from the tuples:

create the extension:

toasted=# create extension pageinspect;
CREATE EXTENSION

and query the user table to get the information:

toasted=# select
page_item_attrs.t_ctid,
page_item_attrs.t_attrs[2],
substr(substr(page_item_attrs.t_attrs[2],octet_length(page_item_attrs.t_attrs[2])-7,4)::text,3) as substr_for_chunk_id,
('x'||regexp_replace(substr(substr(page_item_attrs.t_attrs[2],octet_length(page_item_attrs.t_attrs[2])-7,4)::text,3),'(\w\w)(\w\w)(\w\w)(\w\w)','\4\3\2\1'))::bit(32)::int as chunk_id,
substr(substr(page_item_attrs.t_attrs[2],octet_length(page_item_attrs.t_attrs[2])-3,4)::text,3) as substr_for_toast_relid,
('x'||regexp_replace(substr(substr(page_item_attrs.t_attrs[2],octet_length(page_item_attrs.t_attrs[2])-3,4)::text,3),'(\w\w)(\w\w)(\w\w)(\w\w)','\4\3\2\1'))::bit(32)::int as toast_relid
FROM
generate_series(0, pg_relation_size('bdttab'::regclass::text) / 8192 - 1) blkno ,
heap_page_item_attrs(get_raw_page('bdttab', blkno::int), 'bdttab'::regclass) as page_item_attrs
where
substr(page_item_attrs.t_attrs[2]::text,3,2)='01';
 t_ctid |                t_attrs                 | substr_for_chunk_id | chunk_id | substr_for_toast_relid | toast_relid
--------+----------------------------------------+---------------------+----------+------------------------+-------------
 (0,1)  | \x0112142700001027000077f1020074f10200 | 77f10200            |   192887 | 74f10200               |      192884
 (0,2)  | \x0112142700001027000078f1020074f10200 | 78f10200            |   192888 | 74f10200               |      192884
 (0,3)  | \x0112142700001027000079f1020074f10200 | 79f10200            |   192889 | 74f10200               |      192884
 (0,4)  | \x011214270000102700007af1020074f10200 | 7af10200            |   192890 | 74f10200               |      192884
 (0,5)  | \x011214270000102700007bf1020074f10200 | 7bf10200            |   192891 | 74f10200               |      192884
 (0,6)  | \x011214270000102700007cf1020074f10200 | 7cf10200            |   192892 | 74f10200               |      192884
 (0,7)  | \x011214270000102700007df1020074f10200 | 7df10200            |   192893 | 74f10200               |      192884
 (0,8)  | \x011214270000102700007ef1020074f10200 | 7ef10200            |   192894 | 74f10200               |      192884
 (0,9)  | \x011214270000102700007ff1020074f10200 | 7ff10200            |   192895 | 74f10200               |      192884
 (0,10) | \x0112142700001027000080f1020074f10200 | 80f10200            |   192896 | 74f10200               |      192884
(10 rows)

as you can see we are able to get the chunk_id and the toast relation id from the tuples.

Let’s verify that those values make sense, first checking the toast relation id:

toasted=# select relname from pg_class where oid=192884;
     relname
-----------------
 pg_toast_192881
(1 row)

and then checking the chunk_ids from the toast itself:

toasted=# select distinct(chunk_id) from pg_toast.pg_toast_192881;
 chunk_id
----------
   192894
   192895
   192889
   192888
   192893
   192892
   192896
   192891
   192887
   192890
(10 rows)

The values match the ones we got directly from the user table tuples.

Retrieve the chunk_id and chunk_seq directly from the toast index

The toast index contains those information, so let’s query it too:

toasted=# select
page_items.ctid,
page_items.data,
('x'||regexp_replace(substr(page_items.data,1,11),'(\w\w) (\w\w) (\w\w) (\w\w)','\4\3\2\1'))::bit(32)::int as chunk_id,
('x'||regexp_replace(substr(page_items.data,13,23),'(\w\w) (\w\w) (\w\w) (\w\w)','\4\3\2\1'))::bit(32)::int as chunk_seq
FROM
generate_series(1, pg_relation_size('pg_toast.pg_toast_192881_index'::regclass::text) / 8192 - 1) blkno ,
bt_page_items('pg_toast.pg_toast_192881_index', blkno::int) as page_items;

  ctid  |          data           | chunk_id | chunk_seq
--------+-------------------------+----------+-----------
 (0,1)  | 77 f1 02 00 00 00 00 00 |   192887 |         0
 (0,2)  | 77 f1 02 00 01 00 00 00 |   192887 |         1
 (0,3)  | 77 f1 02 00 02 00 00 00 |   192887 |         2
 (0,4)  | 77 f1 02 00 03 00 00 00 |   192887 |         3
 (1,1)  | 77 f1 02 00 04 00 00 00 |   192887 |         4
 (1,2)  | 77 f1 02 00 05 00 00 00 |   192887 |         5
 (1,3)  | 78 f1 02 00 00 00 00 00 |   192888 |         0
 (1,4)  | 78 f1 02 00 01 00 00 00 |   192888 |         1
 (2,1)  | 78 f1 02 00 02 00 00 00 |   192888 |         2
 (2,2)  | 78 f1 02 00 03 00 00 00 |   192888 |         3
 (2,3)  | 78 f1 02 00 04 00 00 00 |   192888 |         4
 (2,4)  | 78 f1 02 00 05 00 00 00 |   192888 |         5
 (3,1)  | 79 f1 02 00 00 00 00 00 |   192889 |         0
 (3,2)  | 79 f1 02 00 01 00 00 00 |   192889 |         1
 (3,3)  | 79 f1 02 00 02 00 00 00 |   192889 |         2
 (3,4)  | 79 f1 02 00 03 00 00 00 |   192889 |         3
 (4,1)  | 79 f1 02 00 04 00 00 00 |   192889 |         4
 (4,2)  | 79 f1 02 00 05 00 00 00 |   192889 |         5
 (4,3)  | 7a f1 02 00 00 00 00 00 |   192890 |         0
 (4,4)  | 7a f1 02 00 01 00 00 00 |   192890 |         1
 (5,1)  | 7a f1 02 00 02 00 00 00 |   192890 |         2
 (5,2)  | 7a f1 02 00 03 00 00 00 |   192890 |         3
.
.
.
 (12,3) | 7f f1 02 00 02 00 00 00 |   192895 |         2
 (12,4) | 7f f1 02 00 03 00 00 00 |   192895 |         3
 (13,1) | 7f f1 02 00 04 00 00 00 |   192895 |         4
 (13,2) | 7f f1 02 00 05 00 00 00 |   192895 |         5
 (13,3) | 80 f1 02 00 00 00 00 00 |   192896 |         0
 (13,4) | 80 f1 02 00 01 00 00 00 |   192896 |         1
 (14,1) | 80 f1 02 00 02 00 00 00 |   192896 |         2
 (14,2) | 80 f1 02 00 03 00 00 00 |   192896 |         3
 (14,3) | 80 f1 02 00 04 00 00 00 |   192896 |         4
 (14,4) | 80 f1 02 00 05 00 00 00 |   192896 |         5
(60 rows)

Note that the chunk_ids coming from the index, the user table tuples and the toast itself match.

Use case example: a toast’s page is corrupted and I want to know which tuples from the user table are affected

Say the page 12 of the toast is corrupted, then we could get the affected chunk_id and chunk_seq from the toast index that way (by filtering on the ctid):

toasted=# select
page_items.ctid,
page_items.data,
('x'||regexp_replace(substr(page_items.data,1,11),'(\w\w) (\w\w) (\w\w) (\w\w)','\4\3\2\1'))::bit(32)::int as chunk_id,
('x'||regexp_replace(substr(page_items.data,13,23),'(\w\w) (\w\w) (\w\w) (\w\w)','\4\3\2\1'))::bit(32)::int as chunk_seq
FROM
generate_series(1, pg_relation_size('pg_toast.pg_toast_192881_index'::regclass::text) / 8192 - 1) blkno ,
bt_page_items('pg_toast.pg_toast_192881_index', blkno::int) as page_items where ctid::text like '(12,%';

  ctid  |          data           | chunk_id | chunk_seq
--------+-------------------------+----------+-----------
 (12,1) | 7f f1 02 00 00 00 00 00 |   192895 |         0
 (12,2) | 7f f1 02 00 01 00 00 00 |   192895 |         1
 (12,3) | 7f f1 02 00 02 00 00 00 |   192895 |         2
 (12,4) | 7f f1 02 00 03 00 00 00 |   192895 |         3
(4 rows)

And then look back at the user table tuples that way by filtering on the chunk_id:

toasted=# select
page_item_attrs.t_ctid,
page_item_attrs.t_attrs[2],
substr(substr(page_item_attrs.t_attrs[2],octet_length(page_item_attrs.t_attrs[2])-7,4)::text,3) as substr_for_chunk_id,
('x'||regexp_replace(substr(substr(page_item_attrs.t_attrs[2],octet_length(page_item_attrs.t_attrs[2])-7,4)::text,3),'(\w\w)(\w\w)(\w\w)(\w\w)','\4\3\2\1'))::bit(32)::int as chunk_id,
substr(substr(page_item_attrs.t_attrs[2],octet_length(page_item_attrs.t_attrs[2])-3,4)::text,3) as substr_for_toast_relid,
('x'||regexp_replace(substr(substr(page_item_attrs.t_attrs[2],octet_length(page_item_attrs.t_attrs[2])-3,4)::text,3),'(\w\w)(\w\w)(\w\w)(\w\w)','\4\3\2\1'))::bit(32)::int as toast_relid
FROM
generate_series(0, pg_relation_size('bdttab'::regclass::text) / 8192 - 1) blkno ,
heap_page_item_attrs(get_raw_page('bdttab', blkno::int), 'bdttab'::regclass) as page_item_attrs
where
substr(page_item_attrs.t_attrs[2]::text,3,2)='01' and ('x'||regexp_replace(substr(substr(page_item_attrs.t_attrs[2],octet_length(page_item_attrs.t_attrs[2])-7,4)::text,3),'(\w\w)(\w\w)(\w\w)(\w\w)','\4\3\2\1'))::bit(32)::int = 192895;

 t_ctid |                t_attrs                 | substr_for_chunk_id | chunk_id | substr_for_toast_relid | toast_relid
--------+----------------------------------------+---------------------+----------+------------------------+-------------
 (0,9)  | \x011214270000102700007ff1020074f10200 | 7ff10200            |   192895 | 74f10200               |      192884
(1 row)

so that we know that this tuple with ctid (0,9) is linked to the corrupted toast page.

Remarks

  • t_attrs[2] is used in the query to retrieve the information from the user table. This is because the TOAST-able field (message) is the 2nd of the relation.
  • substr(page_item_attrs.t_attrs[2]::text,3,2)=’01’ is used to filter the user table tuples information. This is because 0x01 is used as TOASTed string marker (see the slide).

Conclusion

Thanks to pageinspect we have been able to retrieve the chunk_id directly from the user table tuples. We also have been able to link the user table tuples to the toast pages by using user table tuples and toast index data (not querying the toast at all).