Detoast plpgsql variables if they might live across a transaction boundary

Enterprise / PostgreSQL - Tom Lane [sss.pgh.pa.us] - 16 May 2018 18:56 EDT

Up to now, it's been safe for plpgsql to store TOAST pointers in its
variables because the ActiveSnapshot for whatever query called the plpgsql function will surely protect such TOAST values from being vacuumed away, even if the owning table rows are committed dead. With the introduction of procedures, that assumption is no longer good in "non atomic" executions of plpgsql code. We adopt the slightly brute-force solution of detoasting all TOAST pointers at the time they are stored into variables, if we're in a non-atomic context, just in case the owning row goes away.

Some care is needed to avoid long-term memory leaks, since plpgsql tends to run with CurrentMemoryContext pointing to its call-lifespan context, but we shouldn't assume that no memory is leaked by heap_tuple_fetch_attr. In plpgsql proper, we can do the detoasting work in the "eval_mcontext".

Most of the code thrashing here is due to the need to add this capability to expandedrecord.c as well as plpgsql proper. In expandedrecord.c, we can't assume that the caller's context is short-lived, so make use of the short-term sub-context that was already invented for checking domain constraints. In view of this repurposing, it seems good to rename that
variable and associated code from "domain_check_cxt" to "short_term_cxt".

Peter Eisentraut and Tom Lane

Discussion: https://postgr.es/m/5AC06865.9050005@anastigmatix.net

2efc924180 Detoast plpgsql variables if they might live across a transaction boundary.
src/backend/utils/adt/expandedrecord.c | 180 ++++++++++++++++--------
src/include/postgres.h | 2 +
src/include/utils/expandedrecord.h | 14 +-
src/pl/plpgsql/src/pl_exec.c | 74 ++++++++--
src/test/isolation/expected/plpgsql-toast.out | 189 ++++++++++++++++++++++++++
src/test/isolation/isolation_schedule | 1 +
src/test/isolation/specs/plpgsql-toast.spec | 137 +++++++++++++++++++
7 files changed, 522 insertions(+), 75 deletions(-)

Upstream: git.postgresql.org


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