Re: Index Skip Scan

Hi Rafia,

On 6/1/19 6:03 AM, Rafia Sabih wrote:
> Here is my repeatable test case,
> 
> create table t (market text, feedcode text, updated_at timestamptz,
> value float8) ;
> create index on t (market, feedcode, updated_at desc);
> insert into t values('TEST', 'abcdef', (select timestamp '2019-01-10
> 20:00:00' + random() * (timestamp '2014-01-20 20:00:00' - timestamp
> '2019-01-20 20:00:00') ), generate_series(1,100)*9.88);
> insert into t values('TEST', 'jsgfhdfjd', (select timestamp
> '2019-01-10 20:00:00' + random() * (timestamp '2014-01-20 20:00:00' -
> timestamp '2019-01-20 20:00:00') ), generate_series(1,100)*9.88);
> 
> Now, without the patch,
> select distinct on (market, feedcode) market, feedcode from t  where
> market='TEST';
>   market | feedcode
> --------+-----------
>   TEST   | abcdef
>   TEST   | jsgfhdfjd
> (2 rows)
> explain select distinct on (market, feedcode) market, feedcode from t
> where market='TEST';
>                             QUERY PLAN
> ----------------------------------------------------------------
>   Unique  (cost=12.20..13.21 rows=2 width=13)
>     ->  Sort  (cost=12.20..12.70 rows=201 width=13)
>           Sort Key: feedcode
>           ->  Seq Scan on t  (cost=0.00..4.51 rows=201 width=13)
>                 Filter: (market = 'TEST'::text)
> (5 rows)
> 
> And with the patch,
> select distinct on (market, feedcode) market, feedcode from t   where
> market='TEST';
>   market | feedcode
> --------+----------
>   TEST   | abcdef
> (1 row)
> 
> explain select distinct on (market, feedcode) market, feedcode from t
> where market='TEST';
>                                             QUERY PLAN
> ------------------------------------------------------------------------------------------------
>   Index Only Scan using t_market_feedcode_updated_at_idx on t
> (cost=0.14..0.29 rows=2 width=13)
>     Scan mode: Skip scan
>     Index Cond: (market = 'TEST'::text)
> (3 rows)
> 
> Notice that in the explain statement it shows correct number of rows
> to be skipped.
> 

Thanks for your test case; this is very helpful.

For now, I would like to highlight that

  SET enable_indexskipscan = OFF

can be used for testing with the patch applied.

Dmitry and I will look at the feedback provided.

Best regards,
  Jesper