Solved

CLOB update performance

Posted on 2013-01-15
4
919 Views
Last Modified: 2013-04-04
I am updating a CLOB column from a varchar2 column in the same table.  My sql is:


update mytable a
SET ON_NARRATIVE_CLOB=(select RTRIM(EXTRACT(XMLAGG(XMLELEMENT("x", NARRATIVE||chr(32)) order by LINE), '/x/text()').getclobval(),',')
                                                from mytable b
                                                where a.column= b.column
                                                )

I have 1.2 million rows.  The update is very slow.  Any suggestions, comments, direction would be appreciated.  This is a one-time load for now and incrementals after this first load but I would like to get it as fast as possible.  Oracle 11.2.  Should I rewrite with PL/SQL?  Use a staging table?  Thanks.
0
Comment
Question by:diannagibbs
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 2
  • 2
4 Comments
 
LVL 74

Expert Comment

by:sdstuber
ID: 38780249
try this....


CREATE OR REPLACE TYPE vcarray AS TABLE OF varchar2(4000);


CREATE OR REPLACE FUNCTION tbl2clob(p_tbl IN vcarray, p_delimiter IN VARCHAR2 DEFAULT ',')
    RETURN CLOB
    DETERMINISTIC
IS
    v_str    VARCHAR2(32767);
    v_clob   CLOB;
BEGIN
    DBMS_LOB.createtemporary(v_clob, TRUE);

    IF p_tbl.COUNT > 0
    THEN
        v_str  := p_tbl(1);

        FOR i IN 2 .. p_tbl.COUNT
        LOOP
            -- Appending to clobs is slower than appending to varchar2
            -- so use varchar2 until you can't anymore then append one big chunk

            v_str  := v_str || p_delimiter || p_tbl(i);

            IF LENGTH(v_str) > 28700
            THEN
                v_clob  := v_clob || v_str;
                v_str   := NULL;
            END IF;
        END LOOP;
    END IF;

    IF v_str IS NOT NULL
    THEN
        v_clob  := v_clob || v_str;
    END IF;

    RETURN v_clob;
END;
/


UPDATE mytable a
   SET on_narrative_clob      =
           (SELECT tbl2clob(CAST(COLLECT(b.narrative ORDER BY b.line) AS vcarray), ' ')
              FROM mytable b
             WHERE a.column = b.column)
0
 

Author Comment

by:diannagibbs
ID: 38782609
Unfortunately, this is increasing my time by 3.  This is a development database with small SGA.  Very challenging.
0
 
LVL 74

Accepted Solution

by:
sdstuber earned 500 total points
ID: 38782724
how odd,  the COLLECT method is consistently faster for me.

Also, as the data volume grows XMLAGG will eventually hit ORA-31186: Document contains too many nodes

and, if you happen to have any strings of 4000 characters the XMLAGG method will fail
with  ORA-01489: result of string concatenation is too long


Using COLLECT, these errors don't occur.
It's not only faster but, more importantly is able to finish whereas XMLAGG simply fails.
0
 

Author Comment

by:diannagibbs
ID: 38785383
I'll try it again tomorrow.  I finally was able to get the 1.3mill rows to complete but took 3 hours.
0

Featured Post

Free Tool: SSL Checker

Scans your site and returns information about your SSL implementation and certificate. Helpful for debugging and validating your SSL configuration.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Truncate is a DDL Command where as Delete is a DML Command. Both will delete data from table, but what is the difference between these below statements truncate table <table_name> ?? delete from <table_name> ?? The first command cannot be …
When it comes to protecting Oracle Database servers and systems, there are a ton of myths out there. Here are the most common.
Via a live example, show how to take different types of Oracle backups using RMAN.
This video shows how to copy an entire tablespace from one database to another database using Transportable Tablespace functionality.
Suggested Courses

634 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question