26a3c0a45c
The reference page said WAIT "cannot be used in contexts where such a
snapshot must remain active, including transactions running at isolation
levels higher than READ COMMITTED", which reads as a blanket ban at those
levels. It is not one. The rule the code enforces is that the current
transaction must hold no snapshot, and a transaction-snapshot-mode
transaction has no snapshot until its first query: WaitStmt is exempt in
PlannedStmtRequiresSnapshot(), so the command itself does not take one.
WAIT therefore works before that first query, which is the order an
application wants anyway, and it is what 447aae13b0 set out to allow by
keeping the wait snapshot-free.
State the rule in terms of the snapshot rather than the isolation level,
say that WAIT acquires none of its own and name the other ways to hold
one, and give the consequence of holding one: it can delay replay, which
standby_replay waits for and which the other standby modes ultimately
depend on as well.
The error for a held snapshot named the internal snapshot states instead
of saying what the user did, and attached the isolation-level
explanation as DETAIL unconditionally, so a READ COMMITTED session with a
cursor was told its isolation level was too high. Report what is held
rather than how it is held, and emit the DETAIL only when the isolation
level is in fact the reason.
Reported-by: Sami Imseih <samimseih.pg@gmail.com>
Reported-by: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Sami Imseih <samimseih.pg@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/CAN12%2BYJddgAZzA36JZFX%2BYrnNR7Uubp9qWFz6ASs3v0kL30AMA%40mail.gmail.com
Backpatch-through: 19
PostgreSQL Database Management System
This directory contains the source code distribution of the PostgreSQL database management system.
PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings.
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General documentation about this version of PostgreSQL can be found at https://www.postgresql.org/docs/devel/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/devel/installation.html.
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