create table tmptab (pk integer primary key, sk integer);

-- insert ebnough records to make psotgresql optimizer use indexes

insert into tmptab values (generate_series(1, 10000), generate_series(1, 10000));

vacuum analyze;

-- create normal index

create index idx on tmptab(sk);

-- just normal index search

select * from tmptab where sk = 100;

pk | sk

-----+-----

100 | 100

(1 row)

-- make index partial

alter index idx where pk < 1000;

-- select using exact partial index range

select * from tmptab where sk = 100 and pk < 1000;

pk | sk

-----+-----

100 | 100

(1 row)

explain select * from tmptab where sk = 100 and pk < 1000;

QUERY PLAN

------------------------------------------------------------------

Index Scan using idx on tmptab (cost=0.28..8.29 rows=1 width=8)

Index Cond: (sk = 100)

(2 rows)

-- select using subset of partial index range

select * from tmptab where sk = 100 and pk < 200;

pk | sk

-----+-----

100 | 100

(1 row)

explain select * from tmptab where sk = 100 and pk < 200;

QUERY PLAN

------------------------------------------------------------------

Index Scan using idx on tmptab (cost=0.28..8.29 rows=1 width=8)

Index Cond: (sk = 100)

Filter: (pk < 200)

(3 rows)

-- select outside partial index range

select * from tmptab where sk = 100 and pk > 1000;

pk | sk

----+----

(0 rows)

explain select * from tmptab where sk = 100 and pk > 1000;

QUERY PLAN

--------------------------------------------------------

Seq Scan on tmptab (cost=0.00..195.00 rows=1 width=8)

Filter: ((pk > 1000) AND (sk = 100))

(2 rows)

-- select without partial index range

select * from tmptab where sk = 100;

pk | sk

-----+-----

100 | 100

(1 row)

explain select * from tmptab where sk = 100;

QUERY PLAN

--------------------------------------------------------

Seq Scan on tmptab (cost=0.00..170.00 rows=1 width=8)

Filter: (sk = 100)

(2 rows)

-- extend partial index range

alter index idx where pk < 10000;

-- select using exact partial index range

select * from tmptab where sk = 1000 and pk < 10000;

pk | sk

------+------

1000 | 1000

(1 row)

explain select * from tmptab where sk = 1000 and pk < 10000;

QUERY PLAN

------------------------------------------------------------------

Index Scan using idx on tmptab (cost=0.28..8.30 rows=1 width=8)

Index Cond: (sk = 1000)

(2 rows)

-- calculating aggregate within exact partial index range

select count(*) from tmptab where sk < 1000 and pk < 10000;

count

-------

999

(1 row)

explain select count(*) from tmptab where sk < 1000 and pk < 10000;

QUERY PLAN

---------------------------------------------------------------------------------

Aggregate (cost=40.28..40.29 rows=1 width=0)

-> Index Only Scan using idx on tmptab (cost=0.28..37.78 rows=1000 width=0)

Index Cond: (sk < 1000)

(3 rows)

-- reducing partial idex predicate

alter index idx where pk < 9000;

-- select using new exact partial index range and key value belonging to old range

select * from tmptab where sk = 9000 and pk < 9000;

pk | sk

----+----

(0 rows)

explain select * from tmptab where sk = 9000 and pk < 9000;

QUERY PLAN

------------------------------------------------------------------

Index Scan using idx on tmptab (cost=0.29..8.30 rows=1 width=8)

Index Cond: (sk = 9000)

(2 rows)

-- select using exact partial index range

select * from tmptab where sk = 900 and pk < 9000;

pk | sk

-----+-----

900 | 900

(1 row)

explain select * from tmptab where sk = 900 and pk < 9000;

QUERY PLAN

------------------------------------------------------------------

Index Scan using idx on tmptab (cost=0.29..8.30 rows=1 width=8)

Index Cond: (sk = 900)

(2 rows)

drop table tmptab;