On Wed, Mar 29, 2017 at 1:10 PM, Pavan Deolasee
<pavan.deolasee@gmail.com> wrote:
>
> On Wed, Mar 29, 2017 at 12:02 PM, Amit Kapila <amit.kapila16@gmail.com>
> wrote:
>>
>> On Wed, Mar 29, 2017 at 11:52 AM, Amit Kapila <amit.kapila16@gmail.com>
>> wrote:
>
> Then during recheck, we pass already compressed values to
> index_form_tuple(). But my point is, the following code will ensure that we
> don't compress it again. My reading is that the first check for
> !VARATT_IS_EXTENDED will return false if the value is already compressed.
>
You are right. I was confused with previous check of VARATT_IS_EXTERNAL.
>
> TBH I couldn't find why the original index insertion code will always supply
> uncompressed values.
>
Just try by inserting large value of text column ('aaaaaa.....bbb')
upto 2.5K. Then have a breakpoint in heap_prepare_insert and
index_form_tuple, and debug both the functions, you can find out that
even though we compress during insertion in heap, the index will
compress the original value again.
> But even if does, and even if the recheck gets it in
> compressed form, I don't see how we will double-compress that.
>
No as I agreed above, it won't double-compress, but still looks
slightly risky to rely on different set of values passed to
index_form_tuple and then compare them.
> As far as, comparing two compressed values go, I don't see a problem there.
> Exact same compressed values should decompress to exact same value. So
> comparing two compressed values and two uncompressed values should give us
> the same result.
>
Yeah probably you are right, but I am not sure if it is good idea to
compare compressed values.
I think with this new changes in btrecheck, it would appear to be much
costlier as compare to what you have few versions back. I am afraid
that it can impact performance for cases where there are few WARM
updates in chain and many HOT updates as it will run recheck for all
such updates. Did we any time try to measure the performance of cases
like that?
--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com
