Adds index on:
1. reference doctype
2. reference name
*Why not composite index?*
There are three type of queries on this doctype
- filtering ref_doctype - doctype index helps here
- filtering ref_name - name index helps here
- filtering both - name index helps here too. Since it has sufficiently
high cardinality. Composite index wont help in case where ref_doctype
isn't specfied.
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