percona=# CREATE TABLE percona (id int, name varchar(20)); CREATE TABLE percona=# CREATE INDEX percona_id_index ON percona (id); CREATE INDEX percona=# INSERT INTO percona VALUES (1,’avinash’),(2,’vallarapu’),(3,’avi’),; INSERT 0 3 percona=# SELECT id, name, ctid from percona; id | name | ctid —-+———–+——- 1 | avinash | (0,1) 2 | vallarapu | (0,2) 3 | avi | (0,3) (3 rows), percona=# DELETE from percona where id < 3; DELETE 2, After deleting the records, let us see the items inside table/index pages, Table ======= percona=# SELECT t_xmin, t_xmax, tuple_data_split('percona'::regclass, t_data, t_infomask, t_infomask2, t_bits) FROM heap_page_items(get_raw_page('percona', 0)); t_xmin | t_xmax | tuple_data_split ——–+——–+——————————————- 3825 | 3826 | {"\\x01000000","\\x116176696e617368"} 3825 | 3826 | {"\\x02000000","\\x1576616c6c6172617075"} 3825 | 0 | {"\\x03000000","\\x09617669"} (3 rows), Index ======= percona=# SELECT * FROM bt_page_items('percona_id_index', 1); itemoffset | ctid | itemlen | nulls | vars | data ————+——-+———+——-+——+————————- 1 | (0,1) | 16 | f | f | 01 00 00 00 00 00 00 00 2 | (0,2) | 16 | f | f | 02 00 00 00 00 00 00 00 3 | (0,3) | 16 | f | f | 03 00 00 00 00 00 00 00 (3 rows). In simple terms, PostgreSQL maintains both the past image and the latest image of a row in its own Table. It means, UNDO is maintained within each table. And this is done through versioning. If you have a database that seems to be missing its performance marks, take a look at how often you’re running the autovacuum and analyze functions—those settings may be all you need to tweak. Let’s observe the following log to understand that better. On Terminal A : We open a transaction and delete a row without committing it. Removing the bloat from tables like this can actually cause decreased performance because instead of re-using the space that VACUUM marks as available, Postgres has to again allocate more pages to that object from disk first before the data can be added. Each relation apart from hash indexes has an FSM stored in a separate file called
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