Re1 (11/27/2015)
tbl_documents table is having 700 rows and tbl_documentdetails table is having 1.6 millions+ rows.
I have created index on tbl_documentdetails table on invoiceid,orderid,billid column as per sequence used in join condition.
If you (almost) always use invoiceid to do lookups on tbl_documentdetails -- and from this code it seems you probably do -- then try clustering tbl_documentdetails on ( invoiceid, orderid, billid ). That is, that table's clustered index should be on those columns, not just a nonclustered index.
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