Hello everyone,
I’d like to revisit the topic of auto VACUUM’s interaction with stored procedures that perform transactions, with a more technical clarification as suggested earlier.
Let’s consider the behavior of VACUUM and system table updates after transaction commits in procedures that frequently open and commit transactions.
As I understand, statistics updates in PostgreSQL, which VACUUM later analyzes, are performed in pgstat_report_stat, called within db/src/backend/tcop/postgres.c in the PostgresMain function. Specifically:
stats_timeout = pgstat_report_stat(false);if (stats_timeout > 0)
{ if (!get_timeout_active(IDLE_STATS_UPDATE_TIMEOUT)) enable_timeout_after(IDLE_STATS_UPDATE_TIMEOUT, stats_timeout);
}
else
{ /* all stats flushed, no need for the timeout */ if (get_timeout_active(IDLE_STATS_UPDATE_TIMEOUT)) disable_timeout(IDLE_STATS_UPDATE_TIMEOUT, false);
}
Inside procedures, when _SPI_commit is called in db/src/backend/executor/spi.c, the main command responsible for completing a transaction is CommitTransactionCommand(). My question is the following:
- Is it expected behavior that system table updates are deferred until all nested transactions are complete? This would mean that auto VACUUM might not account for dead tuples accumulated during procedure execution until the entire main transaction is finished.
- Is it possible or advisable to call pgstat_report_stat after each CommitTransactionCommand() within procedures so that auto VACUUM can track intermediate changes and prevent an accumulation of dead rows?
- To what extent would this approach be viable in terms of performance and correctness?
I look forward to any insights and advice you can offer on this matter.
Best regards,
Vyacheslav Kirillov
От: David G. Johnston <david.g.johnston@gmail.com>
Отправлено: 21 октября 2024 г. 16:55
Кому: Кириллов Вячеслав
Копия: pgsql-hackers@postgresql.org
Тема: Re: Question about VACUUM behavior with sub-transactions in stored procedures
On Monday, October 21, 2024, Кириллов Вячеслав <
vkirillov@diasoft.ru> wrote:
I have a question regarding the behavior of the auto VACUUM in PostgreSQL in the context of using stored procedures with sub-transactions.
This is a general usage inquiry not suited to discussion on -hackers. We have a -general mailing list to discuss how to use the product. This list is for discussing patches.
Here is the scenario: we have several stored procedures that modify or update table data. These procedures use sub-transactions, which are committed via COMMIT.
This isn’t how sub-transactions work. They are created mainly by save points and are not independently committed (by the user in SQL). What you are using are full transactions.
David J.
