Q

Searching on CLOB columns

I need to search in a text column larger than 4KB. So, I changed the type of text column from VARCHAR2 to CLOB. But I found that I can't search in CLOB columns. Is thare any way to do this? It will be better if I don't have to move this column to another table because all application Java EJB CMP are already created.

CLOBs require that you use the DBMS_LOB package to perform substr/instr type searches. In order to use do any kind of searching within the column, you must first get the locator for the CLOB column, the use the DBMS_LOB.SUBSTR or DBMS_LOB.INSTR function to search and/or pull part of the text from the string.

For example:

declare
    src_lob        CLOB;
    pos            INTEGER := 1;
    buf            VARCHAR2(32000);
BEGIN
    SELECT c_lob
      INTO src_lob
      FROM lob_table
     WHERE key_value = 21;
    buf := DBMS_LOB.SUBSTR(src_lob, 200, pos);
    -- and so on....
END;
This code would retrieve the first 200 characters from the CLOB. INSTR works in a similar way. Take a look at the Oracle8i Supplied PL/SQL Packages Reference for more details.

Let's expand the concept with another example and review the details: First, we assume we have a table named pet_table defined as:

CREATE TABLE pet_table
(pet_name   varchar2(30) PRIMARY KEY,
 pet_info   CLOB,
 pet_breed  varchar2(30),
 pet_owner  varchar2(50) );
What we want to be able to do is to read the pet_table for a given pet_name (the key) and retrieve from the CLOB column (pet_info) the first 200 characters. For example purposes, we'll assume that the first 200 characters always contains the pet's pedigree information.

Any LOB type column (CLOB, BLOB, NCLOB) cannot be retrieved and searched directly as LOB data columns simply store a pointer that must be used to obtain the actual column data. So, our next step is to SELECT from the pet_table and retrieve the row for the pet and retrieve the lob pointer (also called the lob locator). Then using that locator, we will execute the DBMS_LOB.SUBSTR function to pull back the first 200 characters of the actual lob data (stored at the location pointed to by the locator). DBMS_LOB.SUBSTR takes 3 parameters: 1) the lob locator pointing to the CLOB data value to retrieve 2) an integer indicating the amount of data to retrieve from the CLOB and 3) the starting position where SUBSTR should begin.

Our PL/SQL block can then be written as follows:

DECLARE
    src_lob_locator  CLOB;
    pedigree_info    VARCHAR2(200);
BEGIN
    SELECT pet_info
      INTO src_lob_locator
      FROM pet_table
     WHERE pet_name = 'FIDO' ;

    pedigree_info := DBMS_LOB.SUBSTR(src_lob_locator, 200, 1);

    /* Put any code that you need to process the returned information here
*/

END;
The main thing to remember with LOB columns is that you CANNOT retrieve the CLOB column directly like you would a regular VARCHAR2 column. In other words, if you tried to execute:
SELECT substr(pet_info, 1, 200)
  FROM pet_table
 WHERE pet_name = 'FIDO';
This would fail because you must first obtain the lob locator and then use the DBMS_LOB package functions and procedures to examine the contents as shown in the PL/SQL block.

INSTR works in a similar way as SUBSTR in that it needs to have a lob locator, except that using INSTR, you can find a specific pattern within the CLOB instead of just pulling back a specified amount of characters. The Oracle8i Supplied PL/SQL Packages Reference under DBMS_LOB has details and examples on using INSTR.


This was first published in April 2002

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