Attributes and Queries are key features of Haiku. While attributes are useful on their own, to display additional information on a file, for a query on them, they need to be indexed. It puts them into a lookup table, which in turn makes queries lightning fast.
The index is part of the filesystem and is kept for every volume/partition separately.

Indexing commands in Terminal

There are several commands to manage the index:

• lsindex - Displays the indexed attributes on the current volume/partition.

  These are the attributes that are indexed by default:
  

  58 ~ ->lsindex 
  BEOS:APP_SIG 
  MAIL:account 
  MAIL:cc 
  MAIL:chain 
  MAIL:draft 
  MAIL:flags 
  MAIL:from 
  MAIL:name 
  MAIL:pending_chain 
  MAIL:priority 
  MAIL:reply 
  MAIL:status 
  MAIL:subject 
  MAIL:thread 
  MAIL:to 
  MAIL:when 
  META:address 
  META:city 
  META:company 
  META:country 
  META:email 
  META:fax 
  META:group 
  META:hphone 
  META:name 
  META:nickname 
  META:state 
  META:url 
  META:wphone 
  META:zip 
  _signature 
  _status 
  _trk/qrylastchange 
  _trk/recentQuery 
  be:deskbar_item_status 
  last_modified 
  name 
  size 
  

• mkindex - Adds an attribute to the index of a volume/partition.

  Usage: mkindex [options] <attribute> 
  Creates a new index for the specified attribute. 
   
    -d, --volume=PATH     a path on the volume to which the index will be added, 
                          defaults to current volume. 
    -t, --type=TYPE       the type of the attribute being indexed.  One of "int", 
                          "llong", "string", "float", or "double". 
                          Defaults to "string". 
        --copy-from       path to volume to copy the indexes from. 
    -v, --verbose         print information about the index being created 
  
  

  Note: Only new files with that attribute come automatically into the index!
  Existing files have to be added manually by copying them and deleting the originals after that. Alternatively you can use the tool reindex.

• rmindex - Removes an attribute from the index of a volume/partition.

  Usage: rmindex [OPTION]... INDEX_NAME 
   
  Removes the index named INDEX_NAME from a disk volume.  Once this has been 
  done, it will no longer be possible to use the query system to search for 
  files with the INDEX_NAME attribute. 
   
    -d, --volume=PATH     a path on the volume from which the index will be 
                           removed 
    -h, --help            display this help and exit 
    -p, --pattern         INDEX_NAME is a pattern 
    -v, --verbose         print information about the index being removed 
   
  INDEX_NAME is the name of a file attribute. 
   
  If no volume is specified, the volume of the current directory is assumed.