{ "date": "2017-09-23T20:03:20", "slug": "wrong-result-with-multitenant-dba_contraints-and-current_schema", "link": "https://www.dbi-services.com/blog/wrong-result-with-multitenant-dba_contraints-and-current_schema/", "title": { "rendered": "Wrong result with multitenant, dba_contraints and current_schema" }, "content": { "rendered": "

By Franck Pachot

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\nMultitenant architecture is not such a big change and this is why I recommend it when you start a project in 12c or if you upgrade to 12.2 – of course after thoroughly testing your application. However, there is a point where you may encounter problems on dictionary queries, because it is really a big change internally. The dictionary separation has several side effects. You should test carefully the queries you do on the dictionary views to get metadata. Here is an example of a bug I recently encountered.
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\nThis happened with a combination of things you should not do very often, and not in a critical use case: query dictionary for constraints owned by your current schema, when different than the user you connect with.

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I create two users: USER1 and USER2

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SQL> connect sys/oracle@//localhost/PDB1 as sysdba\nConnected.\nSQL> grant dba to USER1 identified by USER1 container=current;\nGrant succeeded.\nSQL> grant dba to USER2 identified by USER2 container=current;\nGrant succeeded.\n
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USER1 owns a table which has a constraint:

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\nSQL> connect USER1/USER1@//localhost/PDB1\nConnected.\nSQL> create table DEMO(dummy constraint pk primary key) as select * from dual;\nTable DEMO created.\n
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USER2 can access to the table either by prefixing it with USER1 or by setting the current_schema to USER1

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\nSQL> connect USER2/USER2@//localhost/PDB1\nConnected.\nSQL> alter session set current_schema=USER1;\nSession altered.\n
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Bug

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Ok, now imagine you want to read constraint metadata for the current schema you have set:

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\nSQL> select sys_context('USERENV','CURRENT_SCHEMA'), a.*\n  2  from sys.dba_constraints a\n  3       where owner = sys_context('USERENV','CURRENT_SCHEMA')\n  4  /\n \nno rows selected\n
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No rows selected is a wrong result here because my current_schema is USER1 and USER1 has constraints:

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\nSQL> select owner,constraint_name\n  2  from sys.dba_constraints a\n  3       where owner = 'USER1'\n  4  /\nOWNER  CONSTRAINT_NAME\n-----  ---------------\nUSER1  PK\n
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So, where’s the problem? Let’s have a look at the execution plan:

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\nSQL_ID  2fghqwz1cktyf, child number 0\n-------------------------------------\nselect sys_context('USERENV','CURRENT_SCHEMA'), a.*  from\nsys.dba_constraints a      where owner =\nsys_context('USERENV','CURRENT_SCHEMA')\n \nPlan hash value: 1258862619\n \n--------------------------------------------------------------------------------------------------------------\n| Id  | Operation                | Name                    | Starts | E-Rows | A-Rows |   A-Time   | Buffers |\n--------------------------------------------------------------------------------------------------------------\n|   0 | SELECT STATEMENT         |                         |      1 |        |      0 |00:00:00.32 |    2656 |\n|   1 |  PARTITION LIST ALL      |                         |      1 |      2 |      0 |00:00:00.32 |    2656 |\n|*  2 |   EXTENDED DATA LINK FULL| INT$INT$DBA_CONSTRAINTS |      2 |      2 |      0 |00:00:00.32 |    2656 |\n--------------------------------------------------------------------------------------------------------------\n \nPredicate Information (identified by operation id):\n---------------------------------------------------\n \n   2 - filter(((\"INT$INT$DBA_CONSTRAINTS\".\"OBJECT_TYPE#\"=4 OR\n              (\"INT$INT$DBA_CONSTRAINTS\".\"OBJECT_TYPE#\"=2 AND \"INT$INT$DBA_CONSTRAINTS\".\"ORIGIN_CON_ID\"=TO_NUMBER(SY\n              S_CONTEXT('USERENV','CON_ID')))) AND \"OWNER\"=SYS_CONTEXT('USERENV','CURRENT_SCHEMA')))\n
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I am in 12.2 and DBA_CONSTRAINTS reads from INT$DBA_CONSTRAINTS which reads from INT$INT$DBA_CONSTRAINTS and in multitenant this view being an extended data view will read from CDB$ROOT and from the current container. This is why we see EXTENDED DATA LINK FULL in the execution plan and up to this point the predicates are correct: “OWNER”=SYS_CONTEXT(‘USERENV’,’CURRENT_SCHEMA’)

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The execution through data link is run on each container with parallel processes: they switch to the container and run the underlying query on the view. But when I look at the sql trace of the parallel process running the query on my PDB I can see that the predicate on OWNER has replaced the SYS_CONTEXT(‘USERENV’,’CURRENT_SCHEMA’) with the hardcoded value:

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\nSELECT /*+ NO_STATEMENT_QUEUING RESULT_CACHE (SYSOBJ=TRUE) OPT_PARAM('_ENABLE_VIEW_PDB', 'FALSE') */ OWNER,CONSTRAINT_NAME,CONSTRAINT_TYPE,TABLE_NAME,OBJECT_TYPE#,SEARCH_CONDITION,SEARCH_CONDITION_VC,R_OWNER,R_CONSTRAINT_NAME,DELETE_RULE,STATUS,DEFERRABLE,DEFERRED,VALIDATED,GENERATED,BAD,RELY,LAST_CHANGE,INDEX_OWNER,INDEX_NAME,INVALID,VIEW_RELATED,ORIGIN_CON_ID FROM NO_COMMON_DATA(SYS.\"INT$INT$DBA_CONSTRAINTS\") \"INT$INT$DBA_CONSTRAINTS\" WHERE (\"INT$INT$DBA_CONSTRAINTS\".\"OBJECT_TYPE#\"=4 OR \"INT$INT$DBA_CONSTRAINTS\".\"OBJECT_TYPE#\"=2 AND \"INT$INT$DBA_CONSTRAINTS\".\"ORIGIN_CON_ID\"=TO_NUMBER('3')) AND \"INT$INT$DBA_CONSTRAINTS\".\"OWNER\"=q'\"USER2\"'\n
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And unfortunately, this value is not the right one: USER2 is my connected user, but not the CURRENT_SCHEMA that I have set. In the same trace, I can see where this value comes from:

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\nselect 'q''\"' || SYS_CONTEXT('USERENV', 'CURRENT_SCHEMA') || '\"''' from sys.dual\n
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but it seems that the current_schema was lost through the call to the parallel process and the PDB switch to my container.

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Workaround

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The problem is easy to workaround. This works:

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\nSQL> select owner,constraint_name\n  2  from sys.dba_constraints a\n  3       where owner = ( select sys_context('USERENV','CURRENT_SCHEMA') from dual )\n  4  /\n \nOWNER  CONSTRAINT_NAME\n-----  ---------------\nUSER1  PK\n
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And anyway, better to get the current schema before and pass it as a bind variable. The bind variables are passed correctly through data link queries:

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\nSQL> variable v varchar2(30)\nSQL> exec select sys_context('USERENV','CURRENT_SCHEMA') into :v from dual;\n \nPL/SQL procedure successfully completed.\n \nSQL> select sys_context('USERENV','CURRENT_SCHEMA'), a.*\n  2  from sys.dba_constraints a\n  3       --where owner = sys_context('USERENV','CURRENT_SCHEMA')\n  4       where owner = :v\n  5  /\n
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So what?

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The multitenant architecture is a real challenge for dictionary views. The dictionary is separated: system metadata in CDB$ROOT and user metadata in PDB. But, because of compatibility with non-CDB architecture, the dictionary views must show both of them, and this is where it becomes complex: what was separated on purpose has now to be merged. And complexity is subject to bugs. If you want to get an idea, have a look at dcore.sql in ORACLE_HOME/rdbms/admin and compare 11g version with 12c ones, with all the evolution in 12.1.0.1, 12.1.0.2 and 12.2.0.1

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Added OCT-17

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I’ve opened a SR and the following bug is logged: Bug 26986472 – WRONG RESULT WHILE USING SYS_CONTEXT(‘USERENV’,’CURRENT_SCHEMA’) IN PDB

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