There was some doubt as for the speed of doing the select count(*) in
PostgreSQL and Oracle.
To that end, I copied the most part of the Oracle table I used before to
Postgres. Although the copy
wasn't complete, the resulting table is already significantly larger
than the table it was copied from. The result still shows that Oracle is
significantly faster:
Oracle result:
SQL> alter system flush buffer_cache;
System altered.
SQL> select /*+ full(NO) noparallel */ count(*) from ni_occurrence no;
COUNT(*)
----------
402062638
Elapsed: 00:03:16.45
Hints are necessary because Oracle table is declared as parallel and I
didn't want the PK index to be used for counting. Oracle has a good
habit of using PK's for counting, if available.
SQL> select bytes/1048576 as MB
2 from user_segments
3 where segment_name='NI_OCCURRENCE';
MB
----------
35329
Elapsed: 00:00:00.85
SQL>
So, oracle stores 402 million records in 35GB and counts them in 3
minutes 16.45 seconds The very same table was partially copied to
Postgres, copying died with ORA-01555 snapshot too old sometimes this
morning. I ran vacuumdb -f -z on the database after the copy completed
and the results are below.
mgogala=# select count(*) from ni_occurrence;
count
-----------
382400476
(1 row)
Time: 221716.466 ms
mgogala=#
mgogala=# select 221/60::real;
?column?
------------------
3.68333333333333
(1 row)
Time: 0.357 ms
mgogala=#
mgogala=# select pg_size_pretty(pg_table_size('ni_occurrence'));
pg_size_pretty
----------------
46 GB
(1 row)
Time: 0.420 ms
mgogala=#
The database wasn't restarted, no caches were flushed, the comparison
was done with a serious advantage for PostgreSQL. Postgres needed 3.68
minutes to complete the count which is about the same Oracle but still
somewhat slower. Also, I am worried about the sizes. Postgres table is
11GB larger than the original, despite having less data. That was an
unfair and unbalanced comparison because Oracle's cache was flushed and
Oracle was artificially restrained to use the full table scan without
the aid of parallelism. Here is the same result, with no hints and the
autotrace on, which shows what happens if I turn the hints off:
SQL> select count(*) from ni_occurrence no;
COUNT(*)
----------
402062638
Elapsed: 00:00:52.61
Execution Plan
----------------------------------------------------------
Plan hash value: 53476935
--------------------------------------------------------------------------------
----------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)|
Time | TQ |IN-OUT| PQ Distrib |
--------------------------------------------------------------------------------
----------------------------------------
| 0 | SELECT STATEMENT | | 1 | 54001 (19)|
00:01:08 | | | |
| 1 | SORT AGGREGATE | | 1 | |
| | | |
| 2 | PX COORDINATOR | | | |
| | | |
| 3 | PX SEND QC (RANDOM) | :TQ10000 | 1 | |
| Q1,00 | P->S | QC (RAND) |
| 4 | SORT AGGREGATE | | 1 | |
| Q1,00 | PCWP | |
| 5 | PX BLOCK ITERATOR | | 402M| 54001 (19)|
00:01:08 | Q1,00 | PCWC | |
| 6 | INDEX FAST FULL SCAN| IDX_NI_OCCURRENCE_PID | 402M|
54001 (19)|
00:01:08 | Q1,00 | PCWP | |
--------------------------------------------------------------------------------
----------------------------------------
It took just 52 seconds to count everything, but Oracle didn't even scan
the table, it scanned a unique index, in parallel. That is the
algorithmic advantage that forced me to restrict the execution plan with
hints. My conclusion is that the speed of the full scan is OK, about the
same as Oracle speed. There are, however, three significant algorithm
advantages on the Oracle's side:
1) Oracle can use indexes to calculate "select count"
2) Oracle can use parallelism.
3) Oracle can use indexes in combination with the parallel processing.
Here are the descriptions:
SQL> desc ni_occurrence
Name Null? Type
----------------------------------------- --------
----------------------------
ID NOT NULL NUMBER(22)
PERMANENT_ID NOT NULL VARCHAR2(12)
CALL_LETTERS NOT NULL VARCHAR2(5)
AIRDATE NOT NULL DATE
DURATION NOT NULL NUMBER(4)
PROGRAM_TITLE VARCHAR2(360)
COST NUMBER(15)
ASSETID NUMBER(12)
MARKET_ID NUMBER
GMT_TIME DATE
ORIG_ST_OCC_ID NUMBER
EPISODE VARCHAR2(450)
IMPRESSIONS NUMBER
SQL>
mgogala=# \d ni_occurrence
Table "public.ni_occurrence"
Column | Type | Modifiers
----------------+-----------------------------+-----------
id | bigint | not null
permanent_id | character varying(12) | not null
call_letters | character varying(5) | not null
airdate | timestamp without time zone | not null
duration | smallint | not null
program_title | character varying(360) |
cost | bigint |
assetid | bigint |
market_id | bigint |
gmt_time | timestamp without time zone |
orig_st_occ_id | bigint |
episode | character varying(450) |
impressions | bigint |
Indexes:
"ni_occurrence_pk" PRIMARY KEY, btree (id)
mgogala=#
Oracle block is 16k, version is 10.2.0.5 RAC, 64 bit (is anybody still
using 32bit db servers?) . Postgres is 9.0.1, 64 bit. Both machines are
running Red Hat 5.5:
[mgogala@lpo-postgres-d01 ~]$ cat /etc/redhat-release
Red Hat Enterprise Linux Server release 5.5 (Tikanga)
[mgogala@lpo-postgres-d01 ~]$
Linux lpo-postgres-d01 2.6.18-194.el5 #1 SMP Tue Mar 16 21:52:39 EDT
2010 x86_64 x86_64 x86_64 GNU/Linux
[mgogala@lpo-postgres-d01 ~]$
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