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    <p>Most (good) optimizers will make a stab at using an index for a LIKE clause where the wild card is not appearing first. If the pattern starts with a wild card, there is much less that they can do.</p> <p>If the index is a B-Tree index, as opposed to a hash index (ISAM systems usually use B-Trees), then the leading characters of the clause can be used to constrain the index search. If the system uses hash indexes, then you can't work on partial strings easily, unless you create separate indexes on the first character, then on the first two characters, then the first three characters, ... of the column. An ISAM system might allow you that flexibility; most SQL systems do not and you would have to create columns of 1, 2, 3, ... characters containing the first 1, 2, 3 ... characters of the part number field.</p> <p><em>Added</em>: Comments ask "which DBMS?", which is fair. I can vouch for IBM Informix Dynamic Server (IDS) and Standard Engine (SE) in any version you can lay hands on. I would expect both IBM DB2 (LUW or z/OS) to do it; I would expect Oracle to do it. Comments indicate that PostgreSQL 8.0 and above does it - subject to caveats. I can't answer of my own knowledge for Sybase, Ingres, MS SQL Server, Firebird or MySQL. There may be caveats associated with every DBMS about when the index can be used.</p> <p>Note that if there is another index that provides selectivity, then that may be used in preference to the one that provides access to the wildcard search.</p>
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    1. POOptimal LIKE search in SQL
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    1. USJonathan Leffler
    1. USJonathan Leffler
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    1. COCan you mention specific DBMS engines? I tried with PostgreSQL and it does not seem to do this optimization.
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    2. COPostgreSQL will do it since 8.0, but only for a btree index, an anchored regex (starting with ^) or LIKE clause with no wildcard at the beginning, and the "C" locale.
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    3. COAn index simply on the PartNumber column will be used for a partial match like this in almost every SQL RDBMS I've used (which is about 8 different DBs, including the most popular).
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      1. USTom H
 

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