On Fri, Jul 2, 2021 at 07:11:15PM +0000, PG Doc comments form wrote:
> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/13/transaction-iso.html
> Description:
>
> 13.2.1. Read Committed Isolation Level
> Read Committed is the default isolation level in PostgreSQL. When a
> transaction uses this isolation level, a SELECT query (without a FOR
> UPDATE/SHARE clause) sees only data committed before the query began; it
> never sees either uncommitted data or changes committed during query
> execution by concurrent transactions. In effect, a SELECT query sees a
> snapshot of the database as of the instant the query begins to run. However,
> SELECT does see the effects of previous updates executed within its own
> transaction, even though they are not yet committed. Also note that two
> successive SELECT commands can see different data, even though they are
> within a single transaction, if other transactions commit changes after the
> first SELECT starts and before the second SELECT starts.
>
> I don't understand this very clearly: " it never sees either uncommitted
> data or changes committed during query execution by concurrent
> transactions." and this statement " Also note that two successive SELECT
> commands can see different data, even though they are within a single
> transaction, if other transactions commit changes after the first SELECT
> starts and before the second SELECT starts."
Uh, what is your question? Does page 11 of this help?
https://momjian.us/main/writings/pgsql/mvcc.pdf#page=11
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Bruce Momjian <bruce@momjian.us> https://momjian.us
EDB https://enterprisedb.com
If only the physical world exists, free will is an illusion.