Re: Horrific time for getting 1 record from an index?

On Mon, Nov 11, 2013 at 4:30 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:

> On Mon, Nov 11, 2013 at 3:28 PM, Jim Nasby <jnasby(at)enova(dot)com> wrote:
>
>> On 11/11/13 4:57 PM, Jeff Janes wrote:
>>
>> On Mon, Nov 11, 2013 at 1:57 PM, Jim Nasby <jnasby(at)enova(dot)com <mailto:
>>> jnasby(at)enova(dot)com>> wrote:
>>> Btree indexes have special code that kill index-tuples when the
>>> table-tuple is dead-to-all, so only the first such query after the mass
>>> deletion becomes vacuum-eligible should be slow, even if a vacuum is not
>>> done. But if there are long running transactions that prevent the dead
>>> rows from going out of scope, nothing can be done until those transactions
>>> go away.
>>>
>>
>> There is? I didn't know that, can you point me at code?
>>
>
>
> git grep "kill_prior_tuple"
>
>
>>
>> BTW, I originally had this, even after multiple queries:
>>
>> Buffers: shared hit=1 read=9476
>>
>
What were the timings like? Upon repeated execution it seems like all the
buffers should be loaded and so be "hit", not "read".

> Then vacuum:
>> INFO: index "page_hits_raw_pkey" now contains 50343572 row versions in
>> 182800 pages
>> DETAIL: 3466871 index row versions were removed.
>> 44728 index pages have been deleted, 35256 are currently reusable.
>>
>> Then...
>>
>> Buffers: shared hit=1 read=4
>>
>> So I suspect a vacuum is actually needed...
>
>
> Hmm. Maybe the kill method doesn't unlink the empty pages from the tree?
>

I verified that this is the case--the empty pages remain linked in the tree
until a vacuum removes them. But walking through empty leaf pages is way
faster than resolving pages full of pointers to dead-to-all tuple, so the
kill code still gives a huge benefit. But of course nothing will do much
good until the transaction horizon advances.

Cheers,

Jeff