# Databases

In IDOL Server, databases define logical sets of documents that are grouped in a particular way. This grouping can be of any type, but typical examples include the source of the document, the current status of the document, or a high-level classification of the document.

All documents must be placed into exactly one database. Usually, the destination database is given by the value of a `DatabaseType` field in the document. If the document does not have this field, IDOL Server indexes it into the database specified by the value of the `DREDBNAME` parameter in the `DREADD` index action. If this value is also not specified, IDOL Server does not index the document.

## Administer Databases

The primary use for databases is to allow administrative actions to act on large sets of documents. These include:

- marking a database as read-only

- expiring or deleting all documents from a database

- ensuring deduplication only occurs against documents in the same database.

Because databases must be listed in the configuration file or explicitly added by using a `DRECREATEDBASE` index action, Micro Focus generally recommends that you keep the number of databases relatively small, typically fewer than 100. The maximum permitted number of databases is 65,534.

You can remove databases by using the `DREREMOVEDBASE` index action, or you can remove all the content from a database by using the `DREDELDBASE` index action.

NOTE: If you attempt to delete a database by removing it from the configuration file when it still contains documents, IDOL Server automatically adds it back into the configuration file, as a read-only database.

As an alternative to using index actions, you can use the Databases page in the Control section of IDOL Admin to administer your databases.

## Databases as Query Filters

One common use of databases is to restrict query results to only those in particular databases, by using the `DatabaseMatch` parameter. Typically, it is optimal to do this when the total number of databases is lower than 50. If the field in question contains many more distinct values than this, it is generally more efficient to set the field as a MatchTypefield, and use the FieldText `MATCH` operator.

## Databases in Distributed Systems

Databases often have particular importance in distributed systems in which you combine multiple Content servers by using a DAH. In such systems, you generally use the databases to ensure that particular queries target only the server or servers that contain that data.

Related Topics

- Virtual Databases

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