Avoid misbehavior when persisting a non-stable cursor

Enterprise / PostgreSQL - Tom Lane [sss.pgh.pa.us] - 8 June 2021 21:50 UTC

PersistHoldablePortal has long assumed that it should store the entire output of the query-to-be-persisted, which requires rewinding and re-reading the output. This is problematic if the query is not stable: we might get different row contents, or even a different number of rows, which'd confuse the cursor state mightily.

In the case where the cursor is NO SCROLL, this is very easy to solve: just store the remaining query output, without any rewinding, and tweak the portal's cursor state to match. Aside from removing the semantic problem, this could be significantly more efficient than storing the whole output.

If the cursor is scrollable, there's not much we can do, but it was already the case that scrolling a volatile query's result was pretty unsafe. We can just document more clearly that getting correct results from that is not guaranteed.

There are already prohibitions in place on using SCROLL with FOR UPDATE/SHARE, which is one way for a SELECT query to have non-stable results. We could imagine prohibiting SCROLL when the query contains volatile functions, but that would be expensive to enforce. Moreover, it could break applications that work just fine, if they have functions that are in fact stable but the user neglected to mark them so. So settle for documenting the hazard.

While this problem has existed in some guise for a long time, it got a lot worse in v11, which introduced the possibility of persisting plpgsql cursors (perhaps implicit ones) even when they violate the rules for what can be marked WITH HOLD. Hence, I've chosen to back-patch to v11 but not further.

Per bug #17050 from Алексей Булгаков.

Discussion: https://postgr.es/m/17050-f77aa827dc85247c@postgresql.org

ba2c6d6cec Avoid misbehavior when persisting a non-stable cursor.
doc/src/sgml/plpgsql.sgml | 9 ++++
doc/src/sgml/ref/declare.sgml | 8 ++--
src/backend/commands/portalcmds.c | 19 ++++++--
.../plpgsql/src/expected/plpgsql_transaction.out | 51 ++++++++++++++++++++++
src/pl/plpgsql/src/sql/plpgsql_transaction.sql | 41 +++++++++++++++++
5 files changed, 122 insertions(+), 6 deletions(-)

Upstream: git.postgresql.org

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