[Xapian-tickets] [Xapian] #304: Calculate LHS weight earlier in AND_MAYBE to avoid calling RHS in some cases

[Xapian]

#304: Calculate LHS weight earlier in AND_MAYBE to avoid calling RHS in some
cases
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 Reporter:  richard      |             Owner:  olly
     Type:  enhancement  |            Status:  new
 Priority:  normal       |         Milestone:
Component:  Matcher      |           Version:  SVN trunk
 Severity:  normal       |        Resolution:
 Keywords:               |        Blocked By:
 Blocking:               |  Operating System:  All
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Comment (by olly):

 In [b290278a85e7d4c953450a499cdf7e64f6172574] I reimplemented
 `AndMaybePostList`.  In the new implementation, `next()` calls `check()`
 on the RHS instead of `skip_to()`, so for a phrase RHS we now only check
 if the RHS matches the current docid from the LHS - we won't keep
 advancing the RHS if it doesn't match there.

 That doesn't render this ticket obsolete though - a tighter bound for the
 RHS which is only correct for current document is potentially useful for
 allowing the RHS to give a short-cut "no" early, such as in this
 situation:

 > Also, if the RHS was a phrase operator, it is possible to determine the
 weight of the phrase before checking if the phrase actually existed - this
 could avoid needing to access the positional information in some cases.

 There are other cases too - an OR subquery on the RHS could determine that
 it can't match with enough weight for a given document after seeing that a
 subset of its subqueries don't match it.

 And indeed this probably would make more sense as a variant on `check()`
 rather than `skip_to()` (or perhaps an extra parameter on `check()`.

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Ticket URL: <https://trac.xapian.org/ticket/304#comment:3>
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