On Mon, Oct 21, 2013 at 10:54 AM, Haribabu kommi
<haribabu.ko...@huawei.com> wrote:
> On 20 October 2013 12:06 Amit Kapila wrote:
>>On Tue, Oct 15, 2013 at 3:37 PM, Haribabu kommi <haribabu.ko...@huawei.com>
>>wrote:
>>> On 12 October 2013 11:30 Tom Lane wrote:
>>>>Haribabu kommi <haribabu.ko...@huawei.com> writes:
>>>
>>>>Another way to look at it is that we want to keep any increments to
>>>>n_dead_tuples that occur after VACUUM takes its snapshot. Maybe we could
>>>>have VACUUM copy the n_dead_tuples value as it exists when VACUUM starts,
>>>>and then send that as the value to subtract when it's done?
>>>
>>> Taking of n_dead_tuples copy and pass the same at the vacuum end to
>>> subtract from table stats may not be correct, as vacuum may not be cleaned
>>> all the dead tuples because of tuple visibility To other transactions. How
>>> about resets the n_dead_tuples as zero if it goes negative because of
>>> errors?
>
>>Wouldn't the way you are planing to change n_dead_tuples create inconsistency
>>for n_live_tuples and n_dead_tuples, because it would have counted non
>>deleted tuples as n_live_tuples as per below code:
>>
>>if (tupgone)
>>{
>>..
>>tups_vacuumed += 1;
>>has_dead_tuples = true;
>>}
>>else
>>{
>>num_tuples += 1;
>>hastup = true;
>>..
>>}
>>
>>So now if we just subtract tuples_deleted from n_dead_tuples, it will count
>>the tuples deleted during vacuum both as live tuples and dead tuples.
>>There is one statistics for dead row version's that cannot be removed
>>(nkeep), if we could use that to estimate total remaining dead tuples, then
>>the solution can be inline with Tom's suggestion (What
>>would make more sense to me is for VACUUM to estimate the number of remaining
>>dead tuples somehow and send that in its message.).
>
> Yes, it's correct. "nkeep" counter have the dead tuples which are recently
> dead and are not vacuumed. The removal of tuples vacuumed from dead tuples
> should be the same as "nkeep" counter.
> So if we remove the nkeep from num_tuples which gives us the proper live
> tuples. How about following statement at the end scan for all blocks.
>
> num_tuples -= nkeep;
Actually what I had in mind was to use nkeep to estimate n_dead_tuples
similar to how num_tuples is used to estimate n_live_tuples. I think
it will match what Tom had pointed in his response (>>>>What would
make more sense to me is for VACUUM to estimate the number
>>>>of remaining dead tuples somehow and send that in its message. However,
>>>>since the whole point here is that we aren't accounting for transactions
>>>>that commit while VACUUM runs, it's not very clear how to do that.)
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com
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