Thread: Query related to Logical Replication using test_decoding and unchanged-toast-datum
Query related to Logical Replication using test_decoding and unchanged-toast-datum
From
Kiran K V
Date:
Hi,
I am currently using PostgreSQL version 16 and the test_decoding plugin to perform logical replication (using replication slots). I have a simple table with integer column and JSON column. When a non-JSON column is updated, the value "unchanged-toast-datum" for the JSON column is obtained. This shows that the value recorded in a TOAST table for a JSON column value has not changed and is as expected. could you please tell me whether PostgreSQL will truly log these values to WAL or not ? If not, what will be entered into WAL for the JSON column if it remains unchanged?
Please let me know. Thank you very much.
Regards,
Kiran
Re: Query related to Logical Replication using test_decoding and unchanged-toast-datum
From
"David G. Johnston"
Date:
On Mon, Jan 6, 2025 at 7:21 AM Kiran K V <kirankv.1982@gmail.com> wrote:
could you please tell me whether PostgreSQL will truly log these values to WAL or not ? If not, what will be entered into WAL for the JSON column if it remains unchanged?
Not quite sure what you are looking for but:
"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). For convenience, pointer datums also store the logical datum size (original uncompressed data length), physical stored size (different if compression was applied), and the compression method used, if any. Allowing for the varlena header bytes, the total size of an on-disk TOAST pointer datum is therefore 18 bytes regardless of the actual size of the represented value."
So the WAL of the main table data will contain 18bytes of data for that column. If the toast data really hasn't changed during the period in question the WAL will not contain any data for the toast relation since no changes to it were made.
David J.