![V i s h a l Kashyap @ [Sai Hertz And Control Systems]](https://www.gravatar.com/avatar/29c0ee63bc686a5531356fd75893fcaf?s=40&d=identicon "1 messages, last Feb 25, 2004, User")
![V i s h a l Kashyap @ [Sai Hertz And Control Systems]](https://www.gravatar.com/avatar/29c0ee63bc686a5531356fd75893fcaf?s=32&d=identicon)
select statement against pg_stats returns inconsistent data
The select statements return different data for
most_commons_vals depending on whether n_distinct is
included in the select clause or not.
I only seem to get the behavior below against int8
columns - but I haven't interated through every
conceivable data type either.
Is this expected behavior or perhaps a bug?
Regards,
Shelby Cain
=========================================================
c1scain=# select version();
version
---------------------------------------------------------------------------------------
PostgreSQL 7.4.1 on i686-pc-cygwin, compiled by GCC
gcc (GCC) 3.3.1 (cygming special)
(1 row)
c1scain=# create table test_table (lastname
varchar(20), firstname varchar(20), userid int8,
testid int8);
CREATE TABLE
c1scain=# insert into test_table values ('cain',
'shelby', random()*10000, random()*10000);
INSERT 13015123 1
c1scain=# insert into test_table values ('cain',
'shelby', random()*10000, random()*10000);
INSERT 13015124 1
c1scain=# insert into test_table values ('cain',
'shelby', random()*10000, random()*10000);
INSERT 13015125 1
c1scain=# insert into test_table values ('cain',
'shelby', random()*10000, random()*10000);
INSERT 13015126 1
c1scain=# insert into test_table values ('cain',
'shelby', random()*10000, random()*10000);
INSERT 13015127 1
c1scain=# insert into test_table (select * from
test_table);
INSERT 0 5
c1scain=# insert into test_table (select * from
test_table);
INSERT 0 10
c1scain=# insert into test_table (select * from
test_table);
INSERT 0 20
c1scain=# insert into test_table (select * from
test_table);
INSERT 0 40
c1scain=# analyze test_table;
ANALYZE
c1scain=# select distinct userid from test_table;
userid
--------
211
2641
4333
7642
8053
(5 rows)
c1scain=# select distinct testid from test_table;
testid
--------
73
834
1399
2315
4511
(5 rows)
c1scain=# select tablename, attname, most_common_vals
from pg_stats where tablename = 'test_table';
tablename | attname | most_common_vals
------------+-----------+---------------------------
test_table | lastname | {cain}
test_table | firstname | {shelby}
test_table | userid | {211,2641,4333,7642,8053}
test_table | testid | {73,834,1399,2315,4511}
(4 rows)
c1scain=# select tablename, attname, n_distinct,
most_common_vals from pg_stats where tablename =
'test_table';
tablename | attname | n_distinct |
most_common_vals
------------+-----------+------------+------------------------------------------------------
test_table | lastname | 1 | {cain}
test_table | firstname | 1 | {shelby}
test_table | userid | 5 |
{211,18610093293568,32822140076032,34587371634688,0}
test_table | testid | 5 |
{73,6008659247104,9942849290240,19374597472256,0}
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Shelby Cain <alyandon@yahoo.com> writes:
The select statements return different data for
most_commons_vals depending on whether n_distinct is
included in the select clause or not.
I only seem to get the behavior below against int8
columns - but I haven't interated through every
conceivable data type either.
Hoo, I'm surprised no one noticed this during 7.4 development/testing.
The problem applies for any datatype that requires double alignment,
which includes int8, float8, and timestamp as well as most of the
geometric types. pg_statistic is declared as using type "anyarray",
and this type really needs to be marked as requiring double alignment
so that arrays of double-aligned datatypes will come out correctly.
The correct source fix is a one-line change in pg_type.h, but this will
not propagate into existing databases without an initdb. It looks like
what you'd need to do to fix an existing database is
-- clear out broken data in pg_statistic
DELETE FROM pg_statistic;
-- this should update 1 row:
UPDATE pg_type SET typalign = 'd' WHERE oid = 2277;
-- this should update 6 rows:
UPDATE pg_attribute SET attalign = 'd' WHERE atttypid = 2277;
-- might be a good idea to start a fresh backend at this point
-- repopulate pg_statistic
ANALYZE;
Ugh :-(
regards, tom lane
Tom Lane wrote:
Hoo, I'm surprised no one noticed this during 7.4 development/testing.
The problem applies for any datatype that requires double alignment,
which includes int8, float8, and timestamp as well as most of the
geometric types. pg_statistic is declared as using type "anyarray",
and this type really needs to be marked as requiring double alignment
so that arrays of double-aligned datatypes will come out correctly.
anyarray has been defined this way since 7.3 -- any concerns there? I
see that back then pg_statistic used text[] instead of anyarray, so
perhaps not.
Joe
Joe Conway <mail@joeconway.com> writes:
anyarray has been defined this way since 7.3 -- any concerns there?
I don't think so --- we weren't trying to use it as an actual column
datatype back then.
7.4 has a problem though :-( ... this is one of the "damn I wish we'd
caught that before release" ones, since it can't easily be fixed without
initdb. Reminds me that I need to get to work on making pg_upgrade
viable again.
regards, tom lane
I don't think so --- we weren't trying to use it as an actual column
datatype back then.7.4 has a problem though :-( ... this is one of the "damn I wish we'd
caught that before release" ones, since it can't easily be fixed without
initdb. Reminds me that I need to get to work on making pg_upgrade
viable again.
Has anyone given any thought as to whether dumping and restoring
pg_statistic is worthwhile?
eg. some sort of ALTER TABLE..SET STATISTICS (1.0, 3.3, 'asdf',....)
command?
Chris
Christopher Kings-Lynne <chriskl@familyhealth.com.au> writes:
Has anyone given any thought as to whether dumping and restoring
pg_statistic is worthwhile?
Why? You can reconstruct it with a simple "ANALYZE" command. Dumping
and restoring would mean nailing down cross-version assumptions about
what it contains, which doesn't seem real forward-looking...
regards, tom lane
Why? You can reconstruct it with a simple "ANALYZE" command. Dumping
and restoring would mean nailing down cross-version assumptions about
what it contains, which doesn't seem real forward-looking...
I seem to recall that people like that kind of thing so that the dump is
really the current state of the database.
Also, I believe big db's like DB2 and Oracle do such a thing.
I just recall it being discussed some time ago...
Chris
![V i s h a l Kashyap @ [Sai Hertz And Control Systems]](https://www.gravatar.com/avatar/29c0ee63bc686a5531356fd75893fcaf?s=42&d=identicon)
Dear Shelby Cain ,
Is this expected behavior or perhaps a bug?
For a novice like me can anyone please tell me
1. Will this effect my application developed on PostgreSQL
2. Will my Application break at some point I heavly use the type of
queries defined in the post.
Would be greatfull for any kinda answers.
--
Best Regards,
Vishal Kashyap
Director / Lead Developer,
Sai Hertz And Control Systems Pvt Ltd,
http://saihertz.rediffblogs.com
Jabber IM: vishalkashyap@jabber.org
ICQ : 264360076
-----------------------------------------------
You yourself, as much as anybody in the entire
universe, deserve your love and affection.
- Buddha
---------------
I am usually called as Vishal Kashyap
and my Girlfriend calls me Vishal CASH UP.
Because everyone loves me as Vishal Kashyap
and my Girlfriend loves me as CASH.
___
//\\\
( 0_0 )
----------------o0o-----o0o---------------------
--- Tom Lane <tgl@sss.pgh.pa.us> wrote:Hoo, I'm surprised no one noticed this during 7.4
development/testing.
The problem applies for any datatype that requires
double alignment,
which includes int8, float8, and timestamp as well
as most of the
geometric types. pg_statistic is declared as using
type "anyarray",
and this type really needs to be marked as requiring
double alignment
so that arrays of double-aligned datatypes will come
out correctly.The correct source fix is a one-line change in
pg_type.h, but this will
not propagate into existing databases without an
initdb. It looks like
what you'd need to do to fix an existing database is-- clear out broken data in pg_statistic
DELETE FROM pg_statistic;
-- this should update 1 row:
UPDATE pg_type SET typalign = 'd' WHERE oid = 2277;
-- this should update 6 rows:
UPDATE pg_attribute SET attalign = 'd' WHERE
atttypid = 2277;
-- might be a good idea to start a fresh backend at
this point
-- repopulate pg_statistic
ANALYZE;Ugh :-(
regards, tom lane
Works for me. Thanks!
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Christopher Kings-Lynne wrote:
Why? You can reconstruct it with a simple "ANALYZE" command. Dumping
and restoring would mean nailing down cross-version assumptions about
what it contains, which doesn't seem real forward-looking...I seem to recall that people like that kind of thing so that the dump is
really the current state of the database.Also, I believe big db's like DB2 and Oracle do such a thing.
I just recall it being discussed some time ago...
I have heard of dumping stats so you are sure your production db has the
same stats as your test database, but with ANALYZE so fast, and our
optimizer so good, I don't see a use case for us.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
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