FGAC and new partition features

It is better to start with minor note then to wait for long without any. Thus my first post in this blog would be about very specific problem.

I have spent some time investigating it before get to the bottom of this miscommunication between SQL statement improving and Fine-Grain Access Control (FGAC). Thus I believe it can be interesting for audience too. Below is the test case that allows to easily reproduce the issue.

Create partitioned table with non-standard partition names.

CREATE TABLE TESTONFGAC
( ID NUMBER(5,0) NOT NULL ENABLE,
PartitionKey NUMBER(5,0) DEFAULT 0 NOT NULL ENABLE,
COMENT VARCHAR2(255)
) TABLESPACE "USERS"
PARTITION BY RANGE (PartitionKey)
(PARTITION "1" VALUES LESS THAN (1) SEGMENT CREATION IMMEDIATE TABLESPACE "USERS" ,
PARTITION "2" VALUES LESS THAN (2) SEGMENT CREATION IMMEDIATE TABLESPACE "USERS" ,
PARTITION "3" VALUES LESS THAN (3) SEGMENT CREATION IMMEDIATE TABLESPACE "USERS" )
/

Create FGAC policy function. For our case it does not matter how complicate would it be, thus create just dummy one.

CREATE OR REPLACE FUNCTION TEST_FGA (object_schema IN VARCHAR2, object_name VARCHAR2) RETURN VARCHAR2
IS
BEGIN
RETURN '1=1';
END;
/

Create the policy on the newly created table using the function

exec DBMS_RLS.ADD_POLICY ( 'ETERPANI', 'TESTONFGAC', 'test_privacy', 'ETERPANI', 'TEST_FGA');

Everything before was just preparations. Now we are running the relatively new (since Oracle 10g) syntaxes.

DELETE FROM ETERPANI.TESTONFGAC PARTITION ("3") WHERE ID=3;

This reasonable simple statement fails with error “ORA-28113: policy predicate has error”. The further diagnostic can be found on oracle trace file.

*** 2011-06-25 01:23:27.188
-------------------------------------------------------------
Error information for ORA-28113:
Logon user : ETERPANI
Table/View : ETERPANI.TESTONFGAC
Policy name : TEST_PRIVACY
Policy function: ETERPANI.TEST_FGA
RLS view :
SELECT "ID","PARTITIONKEY","COMENT" FROM "ETERPANI"."TESTONFGAC" PARTITION (3) "TESTONFGAC" WHERE (1=1)
ORA-14108: illegal partition-extended table name syntax
-------------------------------------------------------------

As we see double quotes around partition name magically disappears.
In case of STANDARD partition naming (e.g. FIRST , SECOND e.t.c) it does not matter but if you choose lowercase naming or names starting from numbers you can get the ORA-28113.

Fancy automatic database schema generation tools frequently create objects in lower-case in this case it can be fatal.

The expected conclusion “little wit in the head makes much work for the feet” – keep your solution standard and simple (if you can).

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Optimiser Statistics and Lower Case Columns

I was contacted recently about a performance problem in an application we support. The query summarises the number of issues that have not been closed. A simplified version is shown below:

SELECT  COUNT(*)
FROM    "issues"
WHERE   "state" IN ('open','new');

There are two things of note here. Firstly your eyes do not deceive you – those are lower case table and column names (more on this later). Secondly the “state” column is indexed yet the query is doing a full table scan on the 400,000 row “issues” table.

The reason for the full scan is skewed “state” data. There are a handful of new and open issues but a heck of a lot of closed ones (and no possibility of setting them all to NULL before you ask). So the index looks unattractive to the optimiser:

400,000 rows / 3 distinct values = rubbish index access

The obvious thing to do is put a histogram on the “state” column. Things did not go quite to plan. My demonstration of this is below (on a 9.2.0.8 database – the same as the application in question).

1) Create a test table with a mixture of upper and lower case columns, notice we have to wrap table and column names in double quotes (“”) to avoid a conversion to upper case.

create table "tab1"(col1 number,"col2" number,"col3" number);

begin
	for i in 1..10 loop
		insert into "tab1" values (i,i,i);
	end loop;
	commit;
end;
/

2) DBMS_STATS also performs upper case conversions

exec dbms_stats.gather_table_stats(null,'tab1');

column column_name format a10
select  tc.table_name,tc.column_name, tc.nullable nulls
,       tc.num_distinct, tc.num_buckets buckets
from    user_tab_columns tc
where tc.table_name = 'tab1'
order by 1,2;

TABLE_NAME   COLUMN_NAM N NUM_DISTINCT    BUCKETS
------------ ---------- - ------------ ----------
tab1         COL1       Y
tab1         col2       Y
tab1         col3       Y

And again with double quotes (“”) to prevent case conversion

exec dbms_stats.gather_table_stats(null,'"tab1"');

column column_name format a10
select  tc.table_name,tc.column_name, tc.nullable nulls
,       tc.num_distinct, tc.num_buckets buckets
from    user_tab_columns tc
where tc.table_name = 'tab1'
order by 1,2;

TABLE_NAME   COLUMN_NAM N NUM_DISTINCT    BUCKETS
------------ ---------- - ------------ ----------
tab1         COL1       Y           10          1
tab1         col2       Y           10          1
tab1         col3       Y           10          1

So far everything works as expected. Now for the histogram.

3) Create histograms on lower and upper case columns

exec dbms_stats.delete_table_stats(null,'"tab1"');
exec dbms_stats.gather_table_stats(null,'"tab1"'-
               ,method_opt=>'for columns col1 size 254');
exec dbms_stats.gather_table_stats(null,'"tab1"'-
               ,method_opt=>'for columns "col2" size 254');

column column_name format a10
select  tc.table_name,tc.column_name, tc.nullable nulls
,       tc.num_distinct, tc.num_buckets buckets
from    user_tab_columns tc
where   tc.table_name = 'tab1'
order by 1,2;

TABLE_NAME   COLUMN_NAM N NUM_DISTINCT    BUCKETS
------------ ---------- - ------------ ----------
tab1         COL1       Y           10          9
tab1         col2       Y
tab1         col3       Y

That wasn’t expected, there’s no histogram on “col2”. After trying an assortment of “special” characters and seeing the following message far too frequently…

ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error
ORA-06512: at "SYS.DBMS_STATS", line 10502
ORA-06512: at "SYS.DBMS_STATS", line 10516
ORA-06512: at line 1

…I did what I should have done originally and searched Metalink:

Bug 4892211 – DBMS_STATS does not recognise lower case / special characters in column names

• This issue is fixed in*
• 10.2.0.3 (Server Patch Set) <javascript:taghelp(‘FIXED_A203’)>
• 11.1.0.6 (Base Release) <javascript:taghelp(‘FIXED_B106’)>

4) At first I thought a workaround would be to add a histogram to all columns on the table and then remove the ones I don’t want. However I’m faced with the same problem – I can’t specify “col3” to remove that histogram:

exec dbms_stats.gather_table_stats(null,'"tab1"'-
               ,method_opt=>'for all columns size 254');

TABLE_NAME   COLUMN_NAM N NUM_DISTINCT    BUCKETS
------------ ---------- - ------------ ----------
tab1         COL1       Y           10          9
tab1         col2       Y           10          9
tab1         col3       Y           10          9

exec dbms_stats.gather_table_stats(null,'"tab1"'-
               ,method_opt=>'for columns "col3" size 1');

TABLE_NAME   COLUMN_NAM N NUM_DISTINCT    BUCKETS
------------ ---------- - ------------ ----------
tab1         COL1       Y           10          1
tab1         col2       Y           10          9
tab1         col3       Y           10          9

So in my case the best I could do was put the histogram on all indexed columns and use “method_opt=>’for all columns size repeat'”for future optimiser statistic collections to keep the histogram in place.