David Rowley <david.rowley@2ndquadrant.com> writes:
> On Mon, 8 Apr 2019 at 02:20, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I like the concept ... but the particular implementation, not so much.
>> It seems way overcomplicated. In the first place, why should we
>> add code to copy entries? Just don't do it except when the table
>> is empty. In the second, I think we could probably have a far
>> cheaper test for how big the table is --- maybe we'd need to
>> expose some function in dynahash.c, but the right way here is just
>> to see how many buckets there are. I don't like adding statistics
>> counting for this, because it's got basically nothing to do with
>> what the actual problem is. (If you acquire and release one lock,
>> and do that over and over, you don't have a bloat problem no
>> matter how many times you do it.)
> hash_get_num_entries() looks cheap enough to me. Can you explain why
> you think that's too expensive?
What I objected to cost-wise was counting the number of lock
acquisitions/releases, which seems entirely beside the point.
We *should* be using hash_get_num_entries(), but only to verify
that the table is empty before resetting it. The additional bit
that is needed is to see whether the number of buckets is large
enough to justify calling the table bloated.
> As cheap as possible sounds good, but I'm confused at why you think
> the table will always be empty at the end of transaction.
It's conceivable that it won't be, which is why we need a test.
I'm simply arguing that if it is not, we can just postpone de-bloating
till it is. Session-level locks are so rarely used that there's no
need to sweat about that case.
regards, tom lane