Xpath Index in PostgreSQL

[Chris Roffler <croffler@xxxxxxxxxxxxx>]

I am trying to setup an index on an xpath _expression_ but the query never uses the index.

Could someone enlighten me please ?

Here is the setup :

CREATE TABLE time_series

(

  id bigint NOT NULL,

  "name" character varying NOT NULL,

  "timestamp" timestamp with time zone NOT NULL,

  start_date timestamp with time zone,

  end_date timestamp with time zone,

.............

  external_attributes xml,

 ..............

)

WITH (

  OIDS=FALSE

);

CREATE INDEX xml_index

  ON time_series

  USING btree

  ((xpath('/AttributeList/Attributes/Attribute/Name/text()'::text, external_attributes)::text[]));

  

And here is the query :

select id, name 

 from 

  time_series 

 where  

(xpath('/AttributeList/Attributes/Attribute/Name/text()', external_attributes))[1]::text='Attribute100'