Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On Wed, Jun 15, 2016 at 1:59 PM, Alvaro Herrera
<alvherre(at)2ndquadrant(dot)com> wrote:
> Kevin Grittner wrote:
>> On Wed, Jun 15, 2016 at 1:50 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>
>>> The expression index case is the one to worry about; if there is a
>>> problem, that's where it is. What bothers me is that a function used
>>> in an expression index could do anything at all - it can read any
>>> table in the database.
>>
>> It *can*, but then you are lying to the database when you call it
>> IMMUTABLE. Such an index can easily become corrupted through
>> normal DML. Without DML the ANALYZE has no problem. So you seem
>> to be concerned that if someone is lying to the database engine to
>> force it accept a function as IMMUTABLE when it actually isn't, and
>> then updating the referenced rows (which is very likely to render
>> the index corrupted), that statistics might also become stale.
>
> We actually go quite some lengths to support this case, even when it's
> the opinion of many that we shouldn't. For example VACUUM doesn't try
> to find index entries using the values in each deleted tuple; instead we
> remember the TIDs and then scan the indexes (possibly many times) to
> find entries that match those TIDs -- which is much slower. Yet we do
> it this way to protect the case that somebody is doing the
> not-really-IMMUTABLE function.
>
> In other words, I don't think we consider the position you argued as
> acceptable.

What are you saying is unacceptable, and what behavior would be
acceptable instead?

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company