ここの記述が怪しいという話 http://d.hatena.ne.jp/Sikushima/20130828/1377663112

explains.sql

-- バージョン
SELECT version();
                                                                    version
-----------------------------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 9.2.0 on x86_64-apple-darwin11.4.2, compiled by Apple clang version 4.0 (tags/Apple/clang-421.0.60) (based on LLVM 3.1svn), 64-bit
(1 row)

-- ランダム文字
CREATE OR REPLACE FUNCTION random_char() RETURNS text AS $$
    SELECT chr((round((random() * 25)::numeric, 0))::int + 97);
$$ LANGUAGE SQL VOLATILE;

-- ランダム文字列
CREATE OR REPLACE FUNCTION random_text(int) RETURNS text AS $$
    SELECT
        string_agg(random_char(), '')
    FROM
        generate_series(1, $1)
$$ LANGUAGE SQL VOLATILE;

-- カラオケ
-- 曲名はランダムな10文字
CREATE TABLE karaoke (
    id int,
    name text
);

INSERT INTO karaoke
SELECT
    i,
    random_text(10)
FROM
    generate_series(1,100000) t(i)
;

ANALYZE karaoke;

-- 曲名リスト
-- karaokeからランダム10件
CREATE TABLE list (
    name text
);

INSERT INTO list
SELECT
    name
FROM
    karaoke
ORDER BY
    random()
LIMIT
    10
;

ANALYZE list;

-- 1
EXPLAIN SELECT * FROM karaoke WHERE name IN ('pchamwvbgw','uqwywbmvdf','knlokuffsp','efoprrpdfx','ibsguemtcj','wfbbqnvjpp','nqcrctclwp','ghrrxihlks','utqipdgehy','uaywepuxxf');

                                                                     QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------
 Seq Scan on karaoke  (cost=0.00..2791.00 rows=10 width=15)
   Filter: (name = ANY ('{pchamwvbgw,uqwywbmvdf,knlokuffsp,efoprrpdfx,ibsguemtcj,wfbbqnvjpp,nqcrctclwp,ghrrxihlks,utqipdgehy,uaywepuxxf}'::text[]))
(2 rows)

-- 2
EXPLAIN SELECT * FROM karaoke WHERE
    name = 'pchamwvbgw'
    OR name = 'uqwywbmvdf'
    OR name = 'knlokuffsp'
    OR name = 'efoprrpdfx'
    OR name = 'ibsguemtcj'
    OR name = 'wfbbqnvjpp'
    OR name = 'nqcrctclwp'
    OR name = 'ghrrxihlks'
    OR name = 'utqipdgehy'
    OR name = 'uaywepuxxf'
;

                                                                                                                                                           QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Seq Scan on karaoke  (cost=0.00..4041.00 rows=10 width=15)
   Filter: ((name = 'pchamwvbgw'::text) OR (name = 'uqwywbmvdf'::text) OR (name = 'knlokuffsp'::text) OR (name = 'efoprrpdfx'::text) OR (name = 'ibsguemtcj'::text) OR (name = 'wfbbqnvjpp'::text) OR (name = 'nqcrctclwp'::text) OR (name = 'ghrrxihlks'::text) OR (name = 'utqipdgehy'::text) OR (name = 'uaywepuxxf'::text))
(2 rows)

-- 3
EXPLAIN SELECT * FROM karaoke WHERE name = 'pchamwvbgw'
UNION ALL SELECT * FROM karaoke WHERE name = 'uqwywbmvdf'
UNION ALL SELECT * FROM karaoke WHERE name = 'knlokuffsp'
UNION ALL SELECT * FROM karaoke WHERE name = 'efoprrpdfx'
UNION ALL SELECT * FROM karaoke WHERE name = 'ibsguemtcj'
UNION ALL SELECT * FROM karaoke WHERE name = 'wfbbqnvjpp'
UNION ALL SELECT * FROM karaoke WHERE name = 'nqcrctclwp'
UNION ALL SELECT * FROM karaoke WHERE name = 'ghrrxihlks'
UNION ALL SELECT * FROM karaoke WHERE name = 'utqipdgehy'
UNION ALL SELECT * FROM karaoke WHERE name = 'uaywepuxxf'
;

                              QUERY PLAN
-----------------------------------------------------------------------
 Result  (cost=0.00..17910.10 rows=10 width=15)
   ->  Append  (cost=0.00..17910.10 rows=10 width=15)
         ->  Seq Scan on karaoke  (cost=0.00..1791.00 rows=1 width=15)
               Filter: (name = 'pchamwvbgw'::text)
         ->  Seq Scan on karaoke  (cost=0.00..1791.00 rows=1 width=15)
               Filter: (name = 'uqwywbmvdf'::text)
         ->  Seq Scan on karaoke  (cost=0.00..1791.00 rows=1 width=15)
               Filter: (name = 'knlokuffsp'::text)
         ->  Seq Scan on karaoke  (cost=0.00..1791.00 rows=1 width=15)
               Filter: (name = 'efoprrpdfx'::text)
         ->  Seq Scan on karaoke  (cost=0.00..1791.00 rows=1 width=15)
               Filter: (name = 'ibsguemtcj'::text)
         ->  Seq Scan on karaoke  (cost=0.00..1791.00 rows=1 width=15)
               Filter: (name = 'wfbbqnvjpp'::text)
         ->  Seq Scan on karaoke  (cost=0.00..1791.00 rows=1 width=15)
               Filter: (name = 'nqcrctclwp'::text)
         ->  Seq Scan on karaoke  (cost=0.00..1791.00 rows=1 width=15)
               Filter: (name = 'ghrrxihlks'::text)
         ->  Seq Scan on karaoke  (cost=0.00..1791.00 rows=1 width=15)
               Filter: (name = 'utqipdgehy'::text)
         ->  Seq Scan on karaoke  (cost=0.00..1791.00 rows=1 width=15)
               Filter: (name = 'uaywepuxxf'::text)
(22 rows)

-- 4
EXPLAIN SELECT * FROM karaoke WHERE name IN (SELECT name FROM list);

                              QUERY PLAN
----------------------------------------------------------------------
 Hash Semi Join  (cost=54.78..1895.15 rows=1990 width=15)
   Hash Cond: (karaoke.name = list.name)
   ->  Seq Scan on karaoke  (cost=0.00..1541.00 rows=100000 width=15)
   ->  Hash  (cost=29.90..29.90 rows=1990 width=11)
         ->  Seq Scan on list  (cost=0.00..29.90 rows=1990 width=11)
(5 rows)

-- exists
EXPLAIN SELECT * FROM karaoke k WHERE EXISTS (SELECT * FROM list l WHERE k.name = l.name);
                               QUERY PLAN
------------------------------------------------------------------------
 Hash Semi Join  (cost=54.78..1895.15 rows=1990 width=15)
   Hash Cond: (k.name = l.name)
   ->  Seq Scan on karaoke k  (cost=0.00..1541.00 rows=100000 width=15)
   ->  Hash  (cost=29.90..29.90 rows=1990 width=11)
         ->  Seq Scan on list l  (cost=0.00..29.90 rows=1990 width=11)
(5 rows)