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  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-79B699F5-5EDE-5561-A385-19ED02248398" xml:lang="en"><title>Indexes</title><shortdesc>Most datatypes can be used in indexes, but there are restrictions
when using <codeph>LongText</codeph> data types.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>Indexes in the store database are B+tree structures with a fixed size key.
This means that text keys may easily generate large indexes, and a maximum
length text column cannot be indexed. </p>
<p>Keys in DBMS may be truncated by specifying the length of a text column
to be used in the index key. Such a key column must always be the last one
in the key, to ensure that the index provides a true ordering. If a text key
column does not supply a truncation length, the full length of the table column
is used.</p>
<p>Apart from <codeph>Binary</codeph> and <codeph>LongBinary</codeph> columns,
all types can be used in an index key including <codeph>LongText8</codeph> and <codeph>LongText16</codeph>,
although these must supply a truncation length (as they have no maximum length)
and must therefore be the last key columns in the key.</p>
</conbody></concept>