BIND 10 #1756: revise "ITERATE" query in sqlite3_accessor

[BIND 10 Development]

#1756: revise "ITERATE" query in sqlite3_accessor
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            Reporter:  jinmei        |                        Owner:
                Type:  defect        |                       Status:  new
            Priority:  major         |                    Milestone:  Year
           Component:  data source   |  3 Task Backlog
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 See https://lists.isc.org/pipermail/bind10-dev/2012-March/003167.html

 With the current schema zone iterator will be slow for a huge zone (I
 actually confirmed it via the command line interface of sqlite3).

 We should either:
 - sort the result only by rname, or
 - add an index on (rname, rdtype)

 The latter has a side effect of making the DB bigger (in my
 experimental data it made the DB file 20% bigger), and will make
 updates slower (I have no data about this though).

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Ticket URL: <http://bind10.isc.org/ticket/1756>
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