Tuning request

Question

I have a query that takes 7 seconds to complete and I want to speed it up by tuning. Any suggestions are appreciated!
Thanks,
Griff
Features of the query:
* All the columns used in the where clause of the select statement have indexes except one CLOB. (A CLOB cannot have an index.)
* The CLOB column is called “text” and is lowered and searched: lower(text) like '%<search_string>%'
* The select statement orders the query, causing a sort.
* The table in the select statement has about 75,000 rows.
I have 1G of RAM and sga_target is set to 804M. The CPU has 2.13 Ghz.
Here's the output from SQL_TRACE and TKPROF:
call     count       cpu    elapsed       disk      query    current        rows
Parse        1      0.00       0.00          0          0          0           0
Execute      1      0.00       0.00          0          0          0           0
Fetch        3      1.52       6.01        848      32668      50057          31
total        5      1.52       6.01        848      32668      50057          31
Misses in library cache during parse: 0
Optimizer mode: ALL_ROWS
Parsing user id: 5 
Rows     Row Source Operation
     31  SORT ORDER BY (cr=32668 pr=848 pw=0 time=6012529 us)
    767   TABLE ACCESS FULL SFBAY_JOBS (cr=32668 pr=848 pw=0 time=8876275 us)

Answer 1

Hi Griff,
You can remove the full-table scan with a %like% with text indexes, but there is overhead. Indexing when using the SQL "like" clause can be tricky because the wildcard "%" operator can invalidate the index. For example a last_name index would be OK with a "like 'SMI%'" query, but unusable with "like '%SMI%'.
One obscure trick for indexing queries "like '%SON'" is to create a REVERSE index and them programmatically reverse the like clause to read "like 'NOS%'", effectively indexing on the other side of the text.
http://www.dba-oracle.com/t_sql_like_clause_index_usage.htm
The Oracle*Text utility (formally called Oracle ConText and Oracle Intermedia) allows us to parse through a large text column and index on the words within the column.
Unlike ordinary b-tree or bitmap indexes, Oracle context, ctxcat and ctxrule indexes are NOT updated as content is changed. Since most standard Oracle databases will use the ctxcat index with standard relational tables, you must decide on a refresh interval.
Hence, Oracle Text indexes are only useful for removing full-table scans when the tables are largely read-only and/or the end-users don’t mind not having 100% search recall:
- The target table is relatively static (e.g. nightly batch updates)
- Your end-users would not mind “missing” the latest row data
Oracle Text works with traditional data columns and also with XML, MS-Word docs and Adobe PDF files that are stored within Oracle. Oracle Text has several index types:
- CTXCAT Indexes - A CTXCAT index is best for smaller text fragments that must be indexed along with other standard relational data (VARCHAR2).
WHERE CATSEARCH(text_column, 'ipod')> 0;
- CONTEXT Indexes - The CONTEXT index type is used to index large amounts of text such as Word, PDF, XML, HTML or plain text documents.
WHERE CONTAINS(test_column, 'ipod', 1) > 0
- CTXRULE Indexes - A CTXRULE index can be used to build document classification applications.
These types of indexes allow you to replace the old-fashioned SQL “LIKE” syntax with “CONTAINS” or “CATSEARCH” SQL syntax:
http://www.dba-oracle.com/oracle_tips_like_sql_index.htm
Hope this helps. . .
Donald K. Burleson
Oracle Press author