[Xapian-tickets] [Xapian] #790: Operators |= and &= with Xapian::Query{}

[Xapian]

#790: Operators |= and &= with Xapian::Query{}
-------------------------+-----------------------------
 Reporter:  dpa-xapian   |             Owner:  olly
     Type:  defect       |            Status:  assigned
 Priority:  normal       |         Milestone:  1.4.12
Component:  Library API  |           Version:  1.4.11
 Severity:  normal       |        Resolution:
 Keywords:               |        Blocked By:
 Blocking:               |  Operating System:  All
-------------------------+-----------------------------
Changes (by olly):

 * status:  new => assigned
 * component:  Other => Library API
 * milestone:  1.4.x => 1.4.12


Comment:

 > I do expect, that Xapian::Query{} constructs an empty query, that is the
 neutral element towards AND and OR operations

 Long, long ago (probably 15-20 years) we tried making a default-
 constructed query work in the context-sensitive way you're expecting, but
 concluded it was more confusing than helpful.

 Now `Xapian::Query{}` works just like `int{}` (which is `0`) in bitwise
 logical operations or `bool{}` (which is `false`) in boolean logical
 operations.  So `Xapian::Query{}` gives you a query which matches nothing
 (and `Xapian::Query::MatchNothing` is just a static instance of this).

 It's easy to handle the case of accumulating onto a query with AND:

 {{{
 if (t.empty()) {
     t = q;
 } else {
     t &= q;
 }
 }}}

 (And `t.empty()` inlines to a NULL pointer test so the extra check is low
 overhead.)

 > q1 |= qOR → q1.get_type() will be OR and the q1 terms will be A and B
 > q2 &= qAND → q2.get_type() will be AND and the q2 terms will be A and B
 >
 > Instead q1.get_type() == q2.get_type() == LEAF_MATCH_NOTHING and contain
 no terms.

 I tested with current RELEASE/1.4 branch (this code hasn't changed since
 1.4.11) and I can't reproduce `q1.get_type() == LEAF_MATCH_NOTHING` -
 instead I get `q1.get_type() == OP_OR` (which is what I'd expect).

 The query operators do simple folding at construction time around
 `Query{}`, so for `q2` the result is simplified to `Query()` rather than
 giving you an `OP_AND` query, and `q1` is actually `qOR`.

 The API documentation for the default `Query` constructor just says
 "Default constructor." which is uselessly vague.  I'll improve that for
 the next release.

--
Ticket URL: <https://trac.xapian.org/ticket/790#comment:1>
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