RE: tracing specific sql_id in 12.2

From: Noveljic Nenad <>
Date: Thu, 14 Dec 2017 16:48:48 +0000
Message-ID: <28387_1513270140_5A32AB7C_28387_2362_1_ECDEF0CC6716EC4596FCBC871F48292AB19229F1_at_ZRH-S231>

No multitenant, and tested with both new and existing 2. session. Most probably a bug - quite inconvenient.

However, I’ve managed to reconstruct the doubly linked list, where the system events are stored:

linked list ptr to root entry:
BCFFDE18+28: *BCFFDE40 = 989423B8 root_Entry:
989423B8+ 0: *989423B8 = 00000080
989423B8+B8: *98942470 = 96DDD200 Traversing linked list:
96DDD198+68: *96DDD200 = 96DDD580
96DDD518+68: *96DDD580 = 98942470

  1. element: 96DDD198+ 0: *96DDD198 = 02160001 96DDD198+28: *96DDD1C0 = 00000006 96DDD198+38: *96DDD1D0 = 00000000 96DDD198+50: *96DDD1E8 = 00000000 96DDD198+68: *96DDD200 = 96DDD580 96DDD198+70: *96DDD208 = 98942470
  2. element: 96DDD518+ 0: *96DDD518 = 0216000E 96DDD518+28: *96DDD540 = 80000001 96DDD518+38: *96DDD550 = 96DDD198 96DDD518+50: *96DDD568 = 96DDD250 96DDD518+68: *96DDD580 = 98942470 96DDD518+70: *96DDD588 = 96DDD200
Explanation of some important locations:
  • BCFFDE18 is the address of x$ksmsp.ksmchcom='dbgdInitEventGr'
  • If the third least significant bit of element’s offset+28 is set to 1, the SQL will be traced. This means that the element 1 should trigger tracing in the example above.
  • The offsets 38 and 50 should contain the pointers to the other element (see the element 2), but curiously for the first element they are not initialized:

96DDD198+38: *96DDD1D0 = 00000000 96DDD198+50: *96DDD1E8 = 00000000 After initializing the offset 0x50 for the first element with the values of the second element the tracing kicked in. (Of course, I did this only in the sandbox environment for experimental purposes.)

The question is, which one in the chain of dbgd functions and under which conditions is changing this location. This might give us a clue about a possible workaround.

The list above was produced with the following Perl program:


From: [] On Behalf Of Tanel Poder Sent: Donnerstag, 14. Dezember 2017 17:15 To: Noveljic Nenad
Cc: ORACLE-L ( Subject: Re: tracing specific sql_id in 12.2

Maybe there's some bug in 12.2 with event propagation when using ALTER SYSTEM, maybe a variation of what's described here:

... Although eventdump seems to show the event is enabled for the session too. Did you start session 2 before or after issuing ALTER SYSTEM in st?

If 12.2 happens to use the Multitenant config, perhaps test running the ALTER SYSTEM at the root CDB level...

Tanel Poder

On Tue, Dec 12, 2017 at 5:02 PM, Noveljic Nenad <<>> wrote: Has somebody tried tracing a specific sql_id in Oracle 12.2? The feature doesn’t seem to work properly.

Here’s the test case:

Session 1:
alter system set events='sql_trace[sql: 2asugy1n1r7y7] level=12' ; SQL> alter system set events='sql_trace[sql: 2asugy1n1r7y7] level=12' ;

System altered.

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Received on Thu Dec 14 2017 - 17:48:48 CET

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