Re: vacuum, performance, and MVCC

On 6/25/2006 12:27 PM, Bruce Momjian wrote:

> Hannu Krosing wrote:
>> > > Maybe we could start from reusing the index tuples which point to
>> > > invisible tuples ? The index is not MVCC anyway, so maybe it is easier
>> > > to do in-place replacement there ?
>> > > 
>> > > This probably has the same obstacles which have prevented us from
>> > > removing those in the first place (removing instead of marking as
>> > > invisible). Does it cause some locking issues ? Or does it go against
>> > > some other constraints of our index lookups ?
>> > > 
>> > > I think that just setting the invisible bit in an index leaf node causes
>> > > nearly as much disk io as removing the node.
>> > > 
>> > > If we could delete/reuse old index tuples, it would solve a sizable
>> > > chunk of index-growth problem, especially for cases where referenced key
>> > > value does not change.
>> > 
>> > I think heap _and_ index reuse is the only useful direction.  Index or
>> > heap reuse alone seems too marginal for the added complexity.
>> 
>> Sure, but index reuse seems a lot easier, as there is nothing additional
>> to remember or clean out when doing it.
> 
> Yes, seems so.  TODO added:
> 
>     * Reuse index tuples that point to heap tuples that are not visible to
>       anyone?
> 
>> When reusing a heap tuple you have to clean out all index entries
>> pointing to it.
> 
> Well, not for UPDATE for no key changes on the same page, if we do that.
> 

An update that results in all the same values of every indexed column of 
a known deleted invisible tuple. This reused tuple can by definition not 
be the one currently updated. So unless it is a table without a primary 
key, this assumes that at least 3 versions of the same row exist within 
the same block. How likely is that to happen?


Jan

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