Обсуждение: 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

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

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
-----------------------------------------------
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universe, deserve your love and affection.
- Buddha
---------------
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and my Girlfriend calls me Vishal CASH UP.
Because everyone loves me as Vishal Kashyap
and my Girlfriend loves me as CASH.
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----------------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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> 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