From: | Michael Lewis <lewis(dot)michaelr(at)gmail(dot)com> |
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To: | PostgreSQL General <pgsql-general(at)lists(dot)postgresql(dot)org> |
Subject: | Mitigating impact of long running read-only queries |
Date: | 2020-12-14 14:41:11 |
Message-ID: | CAMcsB=wKHM-PLWrYy8ZnQ95CitK5X-ZCv+pQWYy9TX5sDxJW=g@mail.gmail.com |
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Lists: | pgsql-general |
https://www.postgresql.org/docs/current/sql-set-transaction.html
"The DEFERRABLE transaction property has no effect unless the transaction
is also SERIALIZABLE and READ ONLY. When all three of these properties are
selected for a transaction, the transaction may block when first acquiring
its snapshot, after which it is able to run without the normal overhead of
a SERIALIZABLE transaction and without any risk of contributing to or being
canceled by a serialization failure. This mode is well suited for
long-running reports or backups."
If I use these three properties together as below, would WAL buildup be
prevented, or will the system still be causing a backup of the WAL for
logical replication during the entire duration of my long reporting query?
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE READ ONLY DEFERRABLE;
--long report query
commit;
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