TAP: Provide access to EXPLAIN pre-query information for Qserv

Target release	DP0.2; user feedback
Epic
Document status	DRAFT
Document owner	Gregory Dubois-Felsmann
Developers
Dependencies	API design; Qserv EXPLAIN capability
Notes	GPDF/FE/CHB consider architecture, in particular whether TAP server is involved and how low priority FE cries to CADC again - extension to TAP standard? not qserv specific

Goals

Give users improved understanding of which queries may execute rapidly vs. go on various long-table-scan queues
Query has access to additional EXPLAIN functionality that we wish to expose to the user

#	Title	User Story	Importance	Notes
1
2

Below is a list of questions to be addressed as a result of this requirements document:

Question	Outcome