In looking at the draft plan... am I right in assuming that at any 'stop' you
can eliminate paths which have consumed the identical set of nodes but are more
expensive?
For instance, at stop 2, the draft shows:
R-N1 (cost: 7.03)
N1-R (cost: 7.31)
R-N2 (cost: 9.08)
N2-R (cost: 9.08)
Since the first two consume both R and N1, will N1-R ever be a better path than
R-N1? If not, then it can be removed from the set, reducing the amount of work
later. Similarly with R-N2 and N2-R. Since they consume the same nodes and
have identical costs, does it matter which one is used? Won't any path
R-N2-x-y-z.. be identical to N2-R-x-y-z?
Maybe I'm over simplifying?
Marc
-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Igor Tandetnik
Sent: Tuesday, April 30, 2013 5:34 PM
To: [email protected]
Subject: Re: [sqlite] Query optimizer bug?
On 4/30/2013 5:26 PM, Ryan Johnson wrote:
> Being able to force certain access methods (use *this* index, not that
> one) would be helpful, though (does "+" do that or just suggest it?).
Unary plus turns a simple column reference (for which an index can be
used) into an expression (which cannot be indexed). This may be used to
suppress the choice of a particular index, at which point the query planner is
likely to choose a different index, often rearranging the joins. So the unary
plus does often influence the query plan, but in a rather roundabout way.
--
Igor Tandetnik
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