Обсуждение: Xpath Index in PostgreSQL

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Xpath Index in PostgreSQL

От
Chris Roffler
Дата:
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'


Re: Xpath Index in PostgreSQL

От
Tom Lane
Дата:
Chris Roffler <croffler@earthlink.net> writes:
> 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 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'

Doesn't work that way --- subscripting isn't an indexable operation.
To make that query fast with a standard index, you'd need the index to
be on
    (xpath('/AttributeList/Attributes/Attribute/Name/text()',
    external_attributes))[1]::text

            regards, tom lane

Re: Xpath Index in PostgreSQL

От
Chris Roffler
Дата:
Thanks for your fast response 

I have the following sql statement now :

CREATE INDEX xml_index
  ON time_series
  USING btree
       (xpath('/AttributeList/Attributes/Attribute/Name/text()',external_attributes))[1]::text

And I am getting the following error :

ERROR:  syntax error at or near "["
LINE 6: ...butes/Attribute/Name/text()',external_attributes))[1]::text;

Any idea ?

Thanks 
Chris
                                                                                      ^


On Fri, Mar 5, 2010 at 3:22 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Chris Roffler <croffler@earthlink.net> writes:
> 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 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'

Doesn't work that way --- subscripting isn't an indexable operation.
To make that query fast with a standard index, you'd need the index to
be on
       (xpath('/AttributeList/Attributes/Attribute/Name/text()',
       external_attributes))[1]::text

                       regards, tom lane

Re: Xpath Index in PostgreSQL

От
Chris Roffler
Дата:
Figured it out , thanks for all your help ( had missing brackets )

Here is the sql statement

CREATE INDEX xml_index
  ON time_series
  USING btree
  ((
  (xpath('/AttributeList/Attributes/Attribute/Name/text()', external_attributes))[1]::text));


Thanks
Chris

On Fri, Mar 5, 2010 at 4:18 PM, Chris Roffler <croffler@earthlink.net> wrote:
Thanks for your fast response 

I have the following sql statement now :

CREATE INDEX xml_index
  ON time_series
  USING btree
       (xpath('/AttributeList/Attributes/Attribute/Name/text()',external_attributes))[1]::text

And I am getting the following error :

ERROR:  syntax error at or near "["
LINE 6: ...butes/Attribute/Name/text()',external_attributes))[1]::text;

Any idea ?

Thanks 
Chris
                                                                                      ^


On Fri, Mar 5, 2010 at 3:22 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Chris Roffler <croffler@earthlink.net> writes:
> 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 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'

Doesn't work that way --- subscripting isn't an indexable operation.
To make that query fast with a standard index, you'd need the index to
be on
       (xpath('/AttributeList/Attributes/Attribute/Name/text()',
       external_attributes))[1]::text

                       regards, tom lane