XML index

I have a table with an xml column, created an index as follows:

*CREATE INDEX xml_index*
* ON test*
* USING btree*
* (((xpath('//*/ChangedBy/text()'::text, external_attributes))[1]::text));*

And here is my select statement:

*Select uuid from t *
* where (xpath('//*/ChangedBy/text()', external_attributes))[1]::text =
'User';*
*
*
I then insert 100rows into this table, then do a select with the above
statement.
*Explain *shows that the query is using the xml_index.

Now I insert 5000 more rows and *Explain *shows that the query does not use
the xml_index anymore.
However, if I drop the index and re create it, then *Explain *tells me that
it's using the index again.

Any ideas what is going on here ?

Thanks
Chris
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