Avoid noninitial substrings

For best performance, avoid filters for noninitial strings. A filter based on a noninitial substring of a column requires the database server to test every value in the column.

SELECT * FROM customer 
   WHERE zipcode[4,5] > '50'

The database server cannot use an index to evaluate such a filter.

The optimizer uses an index to process a filter that tests an initial substring of an indexed column. However, the presence of the substring test can interfere with the use of a composite index to test both the substring column and another column.