Re: Implementation of Flashback Query

From: Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru>
To: pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: Implementation of Flashback Query
Date: 2018-10-17 14:10:58
Message-ID: a1a7e45b-1d16-5e06-5d64-a8a59c1514ec@postgrespro.ru
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On 17.10.2018 06:18, Yang Jie wrote:
> Hello,
>
> We know that in the absence of backups recovery can be cumbersome if
> the data is erroneously deleted or modified and the transaction is
> committed. For this reason, we realize the flashback query function
> based on PostgreSQL10.5, query the misoperated data, at the same time,
> we also want to know the ideas and suggestions of the flashback
> feature, hope that the postgres function is more powerful.
>
> Take a look at the following example:
> # create table test(id int, tx text, ts timestamp);
> # insert into test values (1, 'tx1', now());
> # update test set tx = 'tx2', ts = now();
> # update test set tx = 'tx3', ts = now();
> # update test set tx = 'tx4', ts = now();
>
> # SELECT lp, t_ctid AS ctid, t_xmin AS xmin, t_xmax AS xmax,
> (t_infomask & 128)::boolean AS xmax_is_lock,
> (t_infomask & 1024)::boolean AS xmax_committed
> FROM heap_page_item_attrs(get_raw_page('test', 0), 'test');
> lp | ctid | xmin | xmax | xmax_is_lock | xmax_committed
> ----+-------+------+------+--------------+----------------
> 1 | (0,2) | 6773 | 6774 | f | t
> 2 | (0,3) | 6774 | 6775 | f | t
> 3 | (0,4) | 6775 | 6776 | f | f
> 4 | (0,4) | 6776 | 0 | f | f
> (4 rows)
> # select xmin,xmax,* from test flashback between xid 6773 and 6776;
> xmin | xmax | id | tx | ts
> ------+------+----+-----+----------------------------
> 6773 | 6774 | 1 | tx1 | 2018-10-16 15:43:17.288023
> 6774 | 6775 | 1 | tx2 | 2018-10-16 15:43:40.528367
> 6775 | 6776 | 1 | tx3 | 2018-10-16 15:43:46.943483
> 6776 | 0 | 1 | tx4 | 2018-10-16 15:44:23.295938
> (4 rows)
> # select xmin,xmax,* from test flashback timestamp '2018-10-16 15:43:18';
> xmin | xmax | id | tx | ts
> ------+------+----+-----+----------------------------
> 6773 | 6774 | 1 | tx1 | 2018-10-16 15:43:17.288023
> (1 row)
> # select xmin,xmax,* from test flashback xid 6773;
> xmin | xmax | id | tx | ts
> ------+------+----+-----+----------------------------
> 6773 | 6774 | 1 | tx1 | 2018-10-16 15:43:17.288023
> (1 row)
> # select xmin,xmax,* from test flashback between timestamp '2018-10-16
> 15:43:45' and '2018-10-16 15:43:50';
> xmin | xmax | id | tx | ts
> ------+------+----+-----+----------------------------
> 6774 | 6775 | 1 | tx2 | 2018-10-16 15:43:40.528367
> 6775 | 6776 | 1 | tx3 | 2018-10-16 15:43:46.943483
> (2 rows)
> To achieve:
> MVCC retains an older version of the data item (dead tuple).
> Deferred VACUUM, ensures that misoperated data has not been garbage
> collected.
> vacuum_defer_cleanup_age = 1000000.
> Turn on transaction commit time tracking to ensure that transaction
> commit time is available from xid.
> track_commit_timestamp = on.
> Add the syntax for flashback queries and flashback version queries.
> SELECT column_name FROM table_name.
> FLASHBACK TIMESTAMP/XID expression.
> WHERE condition
> SELECT column_name FROM table_name.
> FLASHBACK BETWEEN TIMESTAMP/XID expression AND expression.
> WHERE condition
> When a flashback query reads data, the appropriate data item is
> selected through the visibility rule.
> Performance:
> The above realizes the function of flashback query, but there is an
> inevitable problem, after setting the vacuum_defer_cleanup_age
> parameter, the, dead tuple accumulates too much, which leads to the
> bloat problem and obvious performance degradation.
> I have the following questions.
> Delayed cleanup, resulting in performance degradation, what are the
> solutions recommended?
> What do you suggest for the flashback feature?
> Although postgres has excellent backup and restore capabilities, have
> you considered adding flashbacks?
> Looking forward to your suggestions and improvements
>
> ---
> Yang Jie
> Highgo Software
> http://www.highgo.com/
>

Hello,

Did you read this thread:

https://www.postgresql.org/message-id/flat/78aadf6b-86d4-21b9-9c2a-51f1efb8a499%40postgrespro.ru

I have also posted patch with prototype implementation of AS OF queries.

--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

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