On Wed, Jun 8, 2016 at 4:04 PM, Kevin Grittner <kgri...@gmail.com> wrote:
> On Wed, Jun 8, 2016 at 2:49 PM, Robert Haas <robertmh...@gmail.com> wrote:
>> Do you have a test case that demonstrates a problem, or an explanation
>> of why you think there is one?
> With old_snapshot_threshold = '1min'
> -- connection 1
> drop table if exists t1;
> create table t1 (c1 int not null);
> insert into t1 select generate_series(1, 1000000);
> begin transaction isolation level repeatable read;
> select 1;
> -- connection 2
> insert into t2 values (1);
> delete from t1 where c1 between 200000 and 299999;
> delete from t1 where c1 = 1000000;
> vacuum analyze verbose t1;
> select pg_sleep_for('2min');
> vacuum analyze verbose t1;  -- repeat if needed until dead rows vacuumed
> -- connection 1
> select c1 from t1 where c1 = 100;
> select c1 from t1 where c1 = 250000;
> The problem occurs when an index is built while an old snapshot
> exists which can't see the effects of early pruning/vacuuming.  The
> fix prevents use of such an index until all snapshots early enough
> to have a problem have been released.

This example doesn't seem to involve any CREATE INDEX or REINDEX
operations, so I'm a little confused.

Generally, I think I see the hazard you're concerned about: I had
failed to realize, as you mentioned upthread, that new index pages
would have an LSN of 0. So if a tuple is pruned early and then the
index is reindexed, old snapshots won't realize that data is missing.
What I'm less certain about is whether you can actually get by with
reusing ii_BrokenHotChain to handle this case.  For example, note this

     * However, when reindexing an existing index, we should do nothing here.
     * Any HOT chains that are broken with respect to the index must predate
     * the index's original creation, so there is no need to change the
     * index's usability horizon.  Moreover, we *must not* try to change the
     * index's pg_index entry while reindexing pg_index itself, and this
     * optimization nicely prevents that.

This logic doesn't apply to the old snapshot case; there, you'd need
to distrust the index whether it was an initial build or a REINDEX,
but it doesn't look like that's what the patch does.  I'm worried
there could be other places where we rely on ii_BrokenHotChain to
detect only one specific condition that isn't quite the same as what
you're trying to use it for here.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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