On 2016-06-15 20:22:24 -0500, Kevin Grittner wrote: > I'm not clear where you see this as being in any way different with > STO. Above it seemed that you saw this as an issue related to > ANALYZE. If there is not early pruning for the table being > analyzed, nothing is at all different. If there is early pruning > the rows are not seen and there could be no detoasting. If there > is a function that lies about IMMUTABLE and reads from a table, it > either functions as before or throws a STO error on page access > (long before any detoasting). Am I missing something?
I'm not sure, I might be missing something myself. Given the frequency of confusion of all senior hackers involved in this discussion... I previously was thinking of this in the context of ANALYZE, but I now think it's a bigger problem (and might not really affect ANALYZE itself). The simplest version of the scenario I'm concerned about is that a tuple in a tuple is *not* vacuumed, even though it's elegible to be removed due to STO. If that tuple has toasted columns, it could be the that the toast table was independently vacuumed (autovac considers main/toast tables separately, or the horizon could change between the two heap scans, or pins could prevent vacuuming of one page, or ...). Toast accesses via tuptoaster.c currently don't perform TestForOldSnapshot_impl(), because they use SnapshotToastData, not SnapshotMVCC. That seems to means two things: 1) You might get 'missing chunk number ...' errors on access to toasted columns. Misleading error, but ok. 2) Because the tuple has been pruned from the toast table, it's possible that the toast oid/va_valueid is reused, because now there's no conflict with GetNewOidWithIndex() anymore. In that case the toast_fetch_datum() might return a tuple from another column & type (all columns in a table share the same toast table), which could lead to almost arbitrary problems. That's not super likely to happen, but could have quite severe consequences once it starts. It seems the easiest way to fix this would be to make TestForOldSnapshot() "accept" SnapshotToast as well. Andres -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
