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Oracle text index tips

Oracle Database Tips by Donald BurlesonFebruary 18, 2015

Question:  How does the Oracle Text index get used to tune SQL?  I hear that Oracle Text indexes can help tune SQL that uses the "like" clause.

Answer:  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 can be set not to update 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. Oracle provides the SYNC operator for this. The default is SY^NC=MANUAL and you must manually synchronize the index with CTX_DDL.SYNC_INDEX.

SYNC (MANUAL | EVERY "interval-string" | ON COMMIT)]

Hence, Oracle Text indexes are most 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 Oracle text indexes allow you to replace the old-fashioned SQL "LIKE" syntax with "CONTAINS" or "CATSEARCH" SQL syntax:

When we execute the query with the new index we see that the full-table scan is replaced with a index scan, greatly reducing execution speed and improving hardware stress:

Execution Plan

Also see these important notes on Oracle SQL "like clause" and index access.  and using Oracle text indexes to remove full-table scans.

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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