Re: WIP: Covering + unique indexes. (the good and the bad)

On 2018-04-07 14:27, Alexander Korotkov wrote:
> On Sat, Apr 7, 2018 at 2:57 PM, Erik Rijkers <er@xs4all.nl> wrote:
> 
>> On 2018-04-06 20:08, Alexander Korotkov wrote:
>> 
>>> [0001-Covering-v15.patch]
>>> 
>> After some more testing I notice there is also a down-side/slow-down 
>> to
>> this patch that is not so bad but more than negligible, and I don't 
>> think
>> it has been mentioned (but I may have missed something in this thread
>> that's now been running for 1.5 year, not to mention the tangential
>> btree-thread(s)).
>> 
>> I attach my test-program, which compares master (this morning) with
>> covered_indexes (warning: it takes a while to generate the used 
>> tables).
>> 
>> The test tables are created as:
>>   create table $t (c1 int, c2 int, c3 int, c4 int);
>>   insert into $t (select x, 2*x, 3*x, 4 from generate_series(1, 
>> $rowcount)
>> as x);
>>   create unique index ${t}uniqueinclude_idx on $t using btree (c1, c2)
>> include (c3, c4);
>> 
>> or for HEAD, just:
>>   create unique index ${t}unique_idx on $t using btree (c1, c2);
>> 
> 
> Do I understand correctly that you compare unique index on (c1, c2) 
> with
> master to unqiue index on (c1, c2) include (c3, c4) with patched 
> version?
> If so then I think it's wrong to say about down-side/slow-down of this
> patch based on this comparison.
> Patch *does not* cause slowdown in this case.  Patch provides user a 
> *new
> option* which has its advantages and disadvantages.  And what you 
> compare
> is advantages and disadvantages of this option, not slow-down of the 
> patch.
> In the case you compare *the same* index on master and patched version,
> then it's possible to say about slow-down of the patch.

OK, I take your point -- you are right.  Although my measurement was (I 
think) correct, my comparison was not (as Teodor wrote, not quite 
'fair').

Sorry, I should have better thought that message through.  The somewhat 
longer time is indeed just a disadvantage of this new option, to be 
balanced against the advantages that are pretty clear too.


Erik Rijkers