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Chapter 3. Changing the search key length in a substring index

By default, the length of the search key for substring indexes must be at least three characters. For example, Directory Server will add the string abc as a search key to an index while ab* will not. However, to improve the search performance, particularly for searches with many wildcard characters, you can shorten the search key length. This increases the number of search keys in the index.

Directory Server has three attributes that change the minimum number of characters required for a search key:

  • nsSubStrBegin: Sets the minimum number of characters for the beginning of a search key, before the wildcard character. For example: 
abc*
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  • nsSubStrMiddle: Sets the minimum number of characters in the search key between wildcard characters. For example: 
*abc*
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  • nsSubStrEnd: Sets the number of characters for the end of a search key, after the wildcard character. For example: 
*xyz
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You can improve search speeds by setting a new search key length for an attribute index.

Procedure

  1. To set new search key length, add the extensibleObject object class and then add the nsSubStrBegin, nsSubStrEnd, or nsSubStrMiddle attributes to the entry. For example: 

    # ldapmodify -D "cn=Directory Manager" -W -H ldap://server.example.com -x

dn: <attribute_name>,cn=index,cn=<database_name>,cn=ldbm database,cn=plugins,cn=config
changetype: modify
add: objectclass
objectclass: extensibleObject
-
add: nsSubStrBegin
nsSubStrBegin: 2
-
add: nsSubStrMiddle
nsSubStrMiddle: 2
-
add: nsSubStrEnd
nsSubStrEnd: 2
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  2. Recreate the index to apply the new setting. For example, while the Directory Server instance is running, use the following command to recreate the index for the specified attribute: 

    # dsconf <instance_name> backend index reindex --attr <attribute_name> <database_name>
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Verification

  1. Select the attribute for which you want to change the search key length, for example, cn.

  2. Dump the cn index:

    1. On an instance with Berkeley Database (BDB), run: 

      # dbscan -D bdb -f /var/lib/dirsrv/slapd-<instance_name>/db/database/cn.db > /tmp/default_len
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    2. On an instance with Lightning Memory-Mapped Database (LMDB), run: 

      # dbscan -D mdb -f /var/lib/dirsrv/slapd-<instance_name>/db/database/cn.db > /tmp/default_len
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  3. Configure the new search key length as described in section Changing the search key length in a substring index using the command line.

  4. Stop the instance to synchronize the database on the disk: 

    # dsctl <instance_name> stop
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    • Dump the cn index:

      1. On an instance with Berkeley Database (BDB), run: 

        # dbscan -D bdb -f /var/lib/dirsrv/slapd-<instance_name>/db/database/cn.db > /tmp/len_2
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      2. On an instance with Lightning Memory-Mapped Database (LMDB), run: 

        # dbscan -D mdb -f /var/lib/dirsrv/slapd-<instance_name>/db/database/cn.db > /tmp/len_2
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  5. Compare len_2 and default_len files: 

    # diff /tmp/len_2 /tmp/default_len
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