Re: Patch: Write Amplification Reduction Method (WARM)

On Thu, Mar 30, 2017 at 5:55 PM, Pavan Deolasee
<pavan.deolasee@gmail.com> wrote:
>
> On Thu, Mar 30, 2017 at 5:27 PM, Amit Kapila <amit.kapila16@gmail.com>
> wrote:
>>
>>
>>
>> How have you verified that?  Have you checked that in
>> heap_prepare_insert it has called toast_insert_or_update() and then
>> returned a tuple different from what the input tup is?  Basically, I
>> am easily able to see it and even the reason why the heap and index
>> tuples will be different.  Let me try to explain,
>> toast_insert_or_update returns a new tuple which contains compressed
>> data and this tuple is inserted in heap where as slot still refers to
>> original tuple (uncompressed one) which is passed to heap_insert.
>> Now, ExecInsertIndexTuples and the calls under it like FormIndexDatum
>> will refer to the tuple in slot which is uncompressed and form the
>> values[] using uncompressed value.
>
>
> Ah, yes. You're right. Not sure why I saw things differently. That doesn't
> anything though because during recheck we'll get compressed value and not do
> anything with it. In the index we already have compressed value and we can
> compare them. Even if we decide to decompress everything and do the
> comparison, that should be possible.
>

I think we should not consider doing compression and decompression as
free at this point in code, because we hold a buffer lock during
recheck. Buffer locks are meant for short-term locks (it is even
mentioned in storage/buffer/README), doing all the
compression/decompression/detoast stuff under these locks doesn't
sound advisable to me.  It can block many concurrent operations.

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com