On Thu, Nov 3, 2016 at 11:00 PM, Thomas Munro <thomas.mu...@enterprisedb.com> wrote: > + /* > + * Avoid log(0)... > + */ > + N = (path->num_workers < 2) ? 2.0 : (double) path->num_workers; > + logN = LOG2(N); > ... > + /* Per-tuple heap maintenance cost */ > + run_cost += path->path.rows * comparison_cost * 2.0 * logN; > > Why multiply by two? The comment above this code says "about log2(N) > comparisons to delete the top heap entry and another log2(N) > comparisons to insert its successor". In fact gather_merge_getnext > calls binaryheap_replace_first, which replaces the top element without > any comparisons at all and then performs a sift-down in log2(N) > comparisons to find its new position. There is no per-tuple "delete" > involved. We "replace" the top element with the value it already had, > just to trigger the sift-down, because we know that our comparator > function might have a new opinion of the sort order of this element. > Very clever! The comment and the 2.0 factor in cost_gather_merge seem > to be wrong though -- or am I misreading the code?
See cost_merge_append, and the header comments threreto. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
