@startDocuBlock post_api_index
@brief creates an index

@RESTHEADER{POST /_api/index#general, Create index, createIndex:general}

@RESTQUERYPARAMETERS

@RESTQUERYPARAM{collection,string,required}
The collection name.

@RESTALLBODYPARAM{index-details,json,required}

@RESTDESCRIPTION
**NOTE** Swagger examples won't work due to the anchor.


Creates a new index in the collection *collection*. Expects
an object containing the index details.

The type of the index to be created must specified in the *type*
attribute of the index details. Depending on the index type, additional
other attributes may need to specified in the request in order to create
the index.

Indexes require the to be indexed attribute(s) in the *fields* attribute
of the index details. Depending on the index type, a single attribute or
multiple attributes can be indexed. In the latter case, an array of
strings is expected.

Indexing the system attribute *_id* is not supported for user-defined indexes. 
Manually creating an index using *_id* as an index attribute will fail with 
an error.

Optionally, an index name may be specified as a string in the *name* attribute.
Index names have the same restrictions as collection names. If no value is
specified, one will be auto-generated.

Some indexes can be created as unique or non-unique variants. Uniqueness
can be controlled for most indexes by specifying the *unique* flag in the
index details. Setting it to *true* will create a unique index.
Setting it to *false* or omitting the *unique* attribute will
create a non-unique index.

**Note**: The following index types do not support uniqueness, and using
the *unique* attribute with these types may lead to an error:

- geo indexes
- fulltext indexes

**Note**: Unique indexes on non-shard keys are not supported in a
cluster.

Hash, skiplist and persistent indexes can optionally be created in a sparse
variant. A sparse index will be created if the *sparse* attribute in
the index details is set to *true*. Sparse indexes do not index documents
for which any of the index attributes is either not set or is *null*. 

The optional attribute **deduplicate** is supported by array indexes of
type *hash* or *skiplist*. It controls whether inserting duplicate index values 
from the same document into a unique array index will lead to a unique constraint
error or not. The default value is *true*, so only a single instance of each
non-unique index value will be inserted into the index per document. Trying to
insert a value into the index that already exists in the index will always fail,
regardless of the value of this attribute.

@RESTRETURNCODES

@RESTRETURNCODE{200}
If the index already exists, then an *HTTP 200* is returned.

@RESTRETURNCODE{201}
If the index does not already exist and could be created, then an *HTTP 201*
is returned.

@RESTRETURNCODE{400}
If an invalid index description is posted or attributes are used that the
target index will not support, then an *HTTP 400* is returned.

@RESTRETURNCODE{404}
If *collection* is unknown, then an *HTTP 404* is returned.
@endDocuBlock