From: "Taylor, Chris David" <ChrisDavid.Taylor@ingrambarge.com>
To: "'Dave.Herring@acxiom.com'" <Dave.Herring@acxiom.com>,	"'oracle-l@freelists.org'" <oracle-l@freelists.org>
Date: Wed, 2 Feb 2011 12:20:18 -0600
Subject: RE: 16kb limit w/ CLOB and LIKE
Thread-Topic: 16kb limit w/ CLOB and LIKE
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Reply-To: ChrisDavid.Taylor@ingrambarge.com

I hadn't considered that LIKE might work on only parts of a field - I would rather it didn't work at all then partially work.
So, I learned something there.

I think you can use some of the DBMS_LOB functions to get at what you want though.

Dbms_lob.instr(clob_field, 'text to search',1,1) > 0, or dbms_lob.substr maybe.


Chris Taylor
Sr. Oracle DBA
Ingram Barge Company
Nashville, TN 37205
Office: 615-517-3355
Cell: 615-663-1673
Email: chris.taylor@ingrambarge.com
 
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-----Original Message-----
From: oracle-l-bounce@freelists.org [mailto:oracle-l-bounce@freelists.org] On Behalf Of Herring Dave - dherri
Sent: Tuesday, February 01, 2011 1:37 PM
To: oracle-l@freelists.org
Subject: 16kb limit w/ CLOB and LIKE

This may have been raised before and if so, I apologize for the duplication, but I found what appears to be a 16KB limit when filtering CLOBs off a LIKE comparison (10.2.0.2 on Linux RHEL 4.x).  So if you were to filter on a CLOB column with something similar to "... AND sql_text LIKE '%<blah blah>%' ...", you'll get false matches if "blah" doesn't show up until after 16KB of the CLOB.

I ran into this searching DBA_HIST_SQLTEXT, checking for certain statement patterns.  To prove to me what was going on, I picked a statement that DOES NOT include the string DAVE.BOGUS_TABLE and is over 16KB in length (actually around 27KB).

Here's my test:

CREATE TABLE dch_test_tb (stmt_len NUMBER, sql_text CLOB); TRUNCATE TABLE dch_test_tb;

DECLARE
   v_line_length NUMBER := 100;
   v_offset      NUMBER := 1;
   v_temp_clob   CLOB;
   v_insert_clob CLOB;
BEGIN
   SELECT sql_text INTO v_temp_clob
     FROM dba_hist_sqltext
    WHERE sql_id = '<known SQL_ID mentioned above>';

   WHILE v_offset <= 28000
   LOOP
      v_insert_clob := v_insert_clob || TO_CLOB(DBMS_LOB.SUBSTR(v_temp_clob, v_line_length, v_offset));
      INSERT INTO dch_test_tb VALUES (v_line_length, v_insert_clob);
      COMMIT;
      v_offset := v_offset + 100;
   END LOOP;
   COMMIT;
END;
/

SELECT LENGTH(sql_text)
  FROM dch_test_tb
 WHERE UPPER(sql_text) LIKE 'SELECT%DAVE.BOGUS_TABLE%'
 ORDER BY 1;

-------------------Output--------------------------

16400
16500
16600
16700
	...
27203

This isn't a huge issue now for me, as REGEXP_LIKE works but is slower.  I just thought others should know about this in case they didn't already.

DAVID HERRING
DBA
Acxiom Corporation
EML   dave.herring@acxiom.com
TEL    630.944.4762
MBL   630.430.5988
1501 Opus Pl, Downers Grove, IL 60515, USA WWW.ACXIOM.COM  

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