12c - SQL Text Expansion
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by noreply(at)blogger.com (Thomas Kyte)
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Published on Mon, 1 Jul 2013 13:28:31 +0000
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2013/07/01
16:26 UTC
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Here is another small but very useful new feature in Oracle Database 12c - SQL Text Expansion. It will come in handy in two cases:
- You are asked to tune what looks like a simple query - maybe a two table join with simple predicates. But it turns out the two tables are each views of views of views and so on... In other words, you've been asked to 'tune' a 15 page query, not a two liner.
- You are asked to take a look at a query against tables with VPD (virtual private database) policies. In order words, you have no idea what you are trying to 'tune'.
A new function, EXPAND_SQL_TEXT, in the DBMS_UTILITY package makes seeing what the "real" SQL is quite easy. For example - take the common view ALL_USERS - we can now:
ops$tkyte%ORA12CR1> variable x clob
ops$tkyte%ORA12CR1> begin
2 dbms_utility.expand_sql_text
3 ( input_sql_text => 'select * from all_users',
4 output_sql_text => :x );
5 end;
6 /
PL/SQL procedure successfully completed.
ops$tkyte%ORA12CR1> print x
X
--------------------------------------------------------------------------------
SELECT "A1"."USERNAME" "USERNAME","A1"."USER_ID" "USER_ID","A1"."CREATED" "CREAT
ED","A1"."COMMON" "COMMON" FROM (SELECT "A4"."NAME" "USERNAME","A4"."USER#" "US
ER_ID","A4"."CTIME" "CREATED",DECODE(BITAND("A4"."SPARE1",128),128,'YES','NO') "
COMMON" FROM "SYS"."USER$" "A4","SYS"."TS$" "A3","SYS"."TS$" "A2" WHERE "A4"."DA
TATS#"="A3"."TS#" AND "A4"."TEMPTS#"="A2"."TS#" AND "A4"."TYPE#"=1) "A1"
Now it is easy to see what query is really being executed at runtime - regardless of how many views of views you might have. You can see the expanded text - and that will probably lead you to the conclusion that maybe that 27 table join to 25 tables you don't even care about might better be written as a two table join.
Further, if you've ever tried to figure out what a VPD policy might be doing to your SQL, you know it was hard to do at best. Christian Antognini wrote up a way to sort of see it - but you never get to see the entire SQL statement: http://www.antognini.ch/2010/02/tracing-vpd-predicates/. But now with this function - it becomes rather trivial to see the expanded SQL - after the VPD has been applied. We can see this by setting up a small table with a VPD policy
ops$tkyte%ORA12CR1> create table my_table
2 ( data varchar2(30),
3 OWNER varchar2(30) default USER
4 )
5 /
Table created.
ops$tkyte%ORA12CR1> create or replace
2 function my_security_function( p_schema in varchar2,
3 p_object in varchar2 )
4 return varchar2
5 as
6 begin
7 return 'owner = USER';
8 end;
9 /
Function created.
ops$tkyte%ORA12CR1> begin
2 dbms_rls.add_policy
3 ( object_schema => user,
4 object_name => 'MY_TABLE',
5 policy_name => 'MY_POLICY',
6 function_schema => user,
7 policy_function => 'My_Security_Function',
8 statement_types => 'select, insert, update, delete' ,
9 update_check => TRUE );
10 end;
11 /
PL/SQL procedure successfully completed.
And then expanding a query against it:
ops$tkyte%ORA12CR1> begin
2 dbms_utility.expand_sql_text
3 ( input_sql_text => 'select * from my_table',
4 output_sql_text => :x );
5 end;
6 /
PL/SQL procedure successfully completed.
ops$tkyte%ORA12CR1> print x
X
--------------------------------------------------------------------------------
SELECT "A1"."DATA" "DATA","A1"."OWNER" "OWNER" FROM (SELECT "A2"."DATA" "DATA",
"A2"."OWNER" "OWNER" FROM "OPS$TKYTE"."MY_TABLE" "A2" WHERE "A2"."OWNER"=USER@!)
"A1"
Not an earth shattering new feature - but extremely useful in certain cases. I know I'll be using it when someone asks me to look at a query that looks simple but has a twenty page plan associated with it!
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