On 28.05.2013, at 2:17, John Mudd <[email protected]> wrote:
> Thanks again.
>
> Well, I have two problems with using the CLUSTER option. It's only temporary
> since any updates, depending how much free space is reserved per page,
> requires re-running the CLUSTER. And my primary concern is that it
> arbitrarily gives an unfair advantage to the primary key SELECT. Still, it's
> easy to test so here are the results. The primary key still looses even with
> the CLUSTER. Granted it is close but considering this is now an unfair
> comparison it still doesn't make sense to me. How can a search for a specific
> row that should be fairly straight forward take longer than a search that
> includes an ORDER BY clause?
>
Well, you do just regular index scan because of LIMIT 1.
And now it is just a matter of index size and table organization.
I also don't understand why you consider CLUSTER unfair - the way you populated
the table was natural cluster over my_key.
But it bothers me why my_key is always better. Can you please test it on
different values but the same rows? Because now it is two different tuples and
you count every io.
>
> test=# CLUSTER test_select USING test_select_pkey ;
> CLUSTER
> 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=19.430..19.431 rows=1
> loops=1)
> -> Index Scan using my_key on test_select (cost=0.00..41938.15
> rows=499992 width=21) (actual time=19.428..19.428 rows=1 loops=1)
> Index Cond: (key1 >= 500000)
> Total runtime: 19.526 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=21.070..21.072 rows=1 loops=1)
> Index Cond: (id = 500000)
> Total runtime: 21.178 ms
>
>
>
>
> On Mon, May 27, 2013 at 10:59 AM, Evgeny Shishkin <[email protected]>
> wrote:
>>
>> On May 27, 2013, at 6:35 PM, John Mudd <[email protected]> wrote:
>>
>>> Thanks, that's easy enough to test. Didn't seem to help though.
>>
>> Ok. And if you CLUSTER tables USING PK?
>>
>>>
>>> test=# REINDEX index test_select_pkey;
>>> REINDEX
>>> 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=16.368..16.369
>>> rows=1 loops=1)
>>> -> Index Scan using my_key on test_select (cost=0.00..41981.16
>>> rows=501333 width=21) (actual time=16.366..16.366 rows=1 loops=1)
>>> Index Cond: (key1 >= 500000)
>>> Total runtime: 16.444 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=23.072..23.074 rows=1 loops=1)
>>> Index Cond: (id = 500000)
>>> Total runtime: 23.192 ms
>>>
>>>
>>>
>>>
>>> On Mon, May 27, 2013 at 10:21 AM, Evgeny Shishkin <[email protected]>
>>> wrote:
>>>>
>>>> On May 27, 2013, at 6:02 PM, John Mudd <[email protected]> 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()
>>>> >
>
