7.3. Index Basics¶
GeoMesa will create various indices for a given
SimpleFeatureType schema (see Index Overview). This
allows the execution of a variety of queries in a optimized manner. GeoMesa will make a best effort to determine
the attributes used for indexing. The attributes to use can also be specified as part of the
see Customizing Index Creation for details.
7.3.1. Spatial Index (Z2/XZ2)¶
SimpleFeatureType has a
Geometry-type attribute (
GeoMesa will create a spatial index on that attribute. If there is more than one
the default one will be used. The default geometry is generally specified with a
* prefix in the
SimpleFeatureType string, and is the one returned by
7.3.2. Spatio-temporal Index (Z3/XZ3)¶
SimpleFeatureType has both a
Geometry-type attribute and a
Date attribute, GeoMesa will
create a spatio-temporal index on those attributes. The
Geometry-type attribute used is the same as
for the spatial index, above. The
Date attribute selected will be the first one declared, or can be
set explicitly. See Setting the Indexed Date Attribute for details on setting the indexed date.
7.3.3. ID Index¶
GeoMesa will always create an ID index on
SimpleFeature.getID(), unless explicitly disabled.
7.3.4. Attribute Index¶
Some queries are slow to answer using the default indices. For example, with twitter data you might want to return all tweets for a given user. To speed up this type of query, any attribute in your simple feature type may be indexed individually.
To index an attribute, add an
index key to the attribute descriptor user data with a value of
Accumulo data stores have an additional option to create reduced ‘join’ attribute indices, which can save space. See Attribute Indices for details.
Setting the user data can be done in multiple ways. See Setting Schema Options for more details.
Attribute indices also support a secondary, tiered index structure. This can improve attribute queries that also contain a spatial and/or temporal predicate. Unless configured differently, the default geometry and date attributes will be used to create a secondary Z3 of XZ3 index.
Secondary indices can only be leveraged for equality queries against the primary attribute, e.g.
name = 'bob' can take advantage of a secondary index, but
name ilike 'bo%' and
name > 'bo'
Instead of using the default, different secondary index structures can be configured by specifying the attributes to use. To customize the secondary index, the indices must be configured through Customizing Index Creation. If a geometry and date attribute are specified, the secondary index will be Z3 of XZ3, as appropriate. If just a geometry is specified, the secondary index will be Z2 of XZ2, as appropriate. If just a date is specified, the secondary index will be an ordered temporal index.
For example, all of the following are valid ways to configure an index on a ‘name’ attribute, assuming a geometry attribute named ‘geom’ and a date attribute named ‘dtg’:
String spec = "name:String,dtg:Date,*geom:Point:srid=4326";
SimpleFeatureType sft = SimpleFeatureTypes.createType("mySft", spec);
// enable a default z3 and a default attribute index
// or, enable a default z3 and an attribute index with a Z2 secondary index
// or, enable a default z3 and an attribute index with a temporal secondary index
To prioritize certain attributes over others, see Cardinality Hints.
List type attributes may be indexed, but querying a list-type index may result in duplicate results. If duplicate results are a problem, users should implement their own de-duplication logic for list queries.
7.4. Index Versioning¶
In order to ensure cross-compatibility, each index created by GeoMesa has a version number that identifies the layout of data on disk, which is fixed at the time of creation. Updating GeoMesa versions will provide bug fixes and new features, but will not update existing data to new index formats.
The exact version of an index used for each schema can be read from the
SimpleFeatureType user data,
or by simply examining the name of the index tables created by GeoMesa.
The following versions are available:
The version numbers here may not correspond exactly to schemas created with GeoMesa versions prior to 2.3.0, as each back-end implementation initially had its own versioning scheme. However, the implementation for each index was consistent across back-ends in a given GeoMesa release, so if you know the GeoMesa version you can determine the index format from the tables above. Refer to the archived GeoMesa 2.2.0 documentation to see the back-end-specific index version numbers.
Note that GeoMesa versions prior to 1.2.2 included a geohash index. That index has been replaced with the Z indices and is no longer supported.