Amit Langote <Langote_Amit_f8@lab.ntt.co.jp> writes:
> On 2016/05/16 22:12, Ildar Musin wrote:
>> Could you please tell is
>> it possible that relcache invalidation occurs during SELECT/UPDATE/DELETE
>> query?
> Hmm, I think invalidation would not occur mid-query since it would have
> acquired a lock on the table.
This is incorrect: invalidation can occur anyway (for example, if
autoanalyze updates the relation's statistics). If you are holding
a lock, you can expect that the relation's schema will not change more
than your lock would allow --- but a cache flush and rebuild could happen
underneath you, so keeping a pointer to any subsidiary relcache data
structure is very dangerous.
The two ways that we've dealt with this type of hazard are to copy data
out of the relcache before using it; or to give the relcache the
responsibility of not moving a particular portion of data if it did not
change. From memory, the latter applies to the tuple descriptor and
trigger data, but we've done most other things the first way.
regards, tom lane
