The bsearchtempkey function causes the method chosen for joining data to be a binary search of an ordered list. This is the fastest method for small to medium sets of data. This method is the default for joins that do not have an index, or where the index use is discarded with forcetempkey.
Example:
SELECT a.*, b.*
FROM db.dbo.tab a, db.dbo.tab b
WHERE
a.col_name = b.col_name {bsearchtempkey}
Currently, bsearchtempkey is the default method used to process joins, and so the above query would be equivalent to:
SELECT a.*, b.*
FROM db.dbo.tab a, db.dbo.tab b
WHERE
a.col_name = b.col_name
However, at some time in the future, the default may change or CONNX may heuristically switch between methods if no function is specified.