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."