On May 27, 2013, at 6:02 PM, John Mudd <johnbm...@gmail.com> wrote: > Postgres 9.1.2 on Ubuntu 12.04 > > Any reason why a select by primary key would be slower than a select that > includes an ORDER BY? I was really hoping using the primary key would give me > a boost. >
You created my_key after data loading, and PK was there all the time. If you REINDEX PK, i bet it will be as fast. > I stopped the server and cleared the O/S cache using "sync; echo 3 > > /proc/sys/vm/drop_caches" between the runs. > > > > test=# VACUUM ANALYZE test_select; > VACUUM > > (stopped postgres; reset O/S cache; started postgres) > > test=# explain analyze SELECT * FROM test_select WHERE key1 >= 500000 ORDER > BY key1, key2, key3, id LIMIT 1; > QUERY PLAN > -------------------------------------------------------------------------------------------------------------------------------------- > Limit (cost=0.00..0.08 rows=1 width=21) (actual time=12.599..12.600 rows=1 > loops=1) > -> Index Scan using my_key on test_select (cost=0.00..41895.49 > rows=498724 width=21) (actual time=12.597..12.597 rows=1 loops=1) > Index Cond: (key1 >= 500000) > Total runtime: 12.678 ms > > (stopped postgres; reset O/S cache; started postgres) > > test=# explain analyze SELECT * FROM test_select WHERE id = 500000; > QUERY PLAN > > --------------------------------------------------------------------------------------------------------------------------------- > Index Scan using test_select_pkey on test_select (cost=0.00..8.36 rows=1 > width=21) (actual time=31.396..31.398 rows=1 loops=1) > Index Cond: (id = 500000) > Total runtime: 31.504 ms > > > > Schema: > > test=# \d test_select > Table "public.test_select" > Column | Type | Modifiers > > --------+--------------+---------------------------------------------------------- > id | integer | not null default > nextval('test_select_id_seq'::regclass) > key1 | integer | > key2 | integer | > key3 | integer | > data | character(4) | > Indexes: > "test_select_pkey" PRIMARY KEY, btree (id) > "my_key" btree (key1, key2, key3, id) > > test=# > > > > Sample data: > > test=# SELECT * FROM test_select LIMIT 10; > id | key1 | key2 | key3 | data > ----+--------+--------+--------+------ > 1 | 984966 | 283954 | 772063 | x > 2 | 817668 | 393533 | 924888 | x > 3 | 751039 | 798753 | 454309 | x > 4 | 128505 | 329643 | 280553 | x > 5 | 105600 | 257225 | 710015 | x > 6 | 323891 | 615614 | 83206 | x > 7 | 194054 | 63506 | 353171 | x > 8 | 212068 | 881225 | 271804 | x > 9 | 644180 | 26693 | 200738 | x > 10 | 136586 | 498699 | 554417 | x > (10 rows) > > > > > Here's how I populated the table: > > import psycopg2 > > conn = psycopg2.connect('dbname=test') > > cur = conn.cursor() > > def random_int(): > n = 1000000 > return random.randint(0,n) > > def random_key(): > return random_int(), random_int(), random_int() > > def create_table(): > cur.execute(''' > DROP TABLE IF EXISTS test_select; > > CREATE TABLE test_select ( > id SERIAL PRIMARY KEY, > key1 INTEGER, > key2 INTEGER, > key3 INTEGER, > data char(4) > ); > ''') > conn.commit() > > n = 1000000 > for i in range(n): > cur.execute("INSERT INTO test_select(key1, key2, key3, data) > VALUES(%s, %s, %s, 'x')", random_key()) > conn.commit() > > cur.execute('CREATE INDEX my_key ON test_select(key1, key2, key3, id)') > conn.commit() > > create_table() > -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance