While thinking about add_path_precheck() function, it occurred to me that it
can discard some paths that otherwise would have chance be accepted in this
part of add_path():
/*
* Same pathkeys and outer rels, and fuzzily
* the same cost, so keep just one; to decide
* which, first check rows and then do a fuzzy
* cost comparison with very small fuzz limit.
* (We used to do an exact cost comparison,
* but that results in annoying
* platform-specific plan variations due to
* roundoff in the cost estimates.) If things
* are still tied, arbitrarily keep only the
* old path. Notice that we will keep only
* the old path even if the less-fuzzy
* comparison decides the startup and total
* costs compare differently.
*/
if (new_path->rows < old_path->rows)
remove_old = true; /* new dominates old */
else if (new_path->rows > old_path->rows)
accept_new = false; /* old dominates new */
else if (compare_path_costs_fuzzily(new_path,
The special case is that the path passed to add_path_precheck() has costs
*equal to* those of the old_path. If pathkeys, outer rells and costs are the
same, as summarized in the comment above, I expect add_path_precheck() to
return false. Do I misread anything?
(Maybe the fact that this does not happen too often is that
add_path_precheck() compares the costs exactly, as opposed to the "fuzzy
comparison"?)
--
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de, http://www.cybertec.at
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