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Branimir Lambov commented on CASSANDRA-8915: -------------------------------------------- Yes, what I'm suggesting is a replacement of the priority queue with a custom version that supports {{swapHead}}. It actually doesn't have to be a full PQ implementation, {{swapHead}} is the only operation needed (the initial heap construction can be done by sorting). > Improve MergeIterator performance > --------------------------------- > > Key: CASSANDRA-8915 > URL: https://issues.apache.org/jira/browse/CASSANDRA-8915 > Project: Cassandra > Issue Type: Improvement > Reporter: Branimir Lambov > Assignee: Branimir Lambov > Priority: Minor > > The implementation of {{MergeIterator}} uses a priority queue and applies a > pair of {{poll}}+{{add}} operations for every item in the resulting sequence. > This is quite inefficient as {{poll}} necessarily applies at least {{log N}} > comparisons (up to {{2log N}}), and {{add}} often requires another {{log N}}, > for example in the case where the inputs largely don't overlap (where {{N}} > is the number of iterators being merged). > This can easily be replaced with a simple custom structure that can perform > replacement of the top of the queue in a single step, which will very often > complete after a couple of comparisons and in the worst case scenarios will > match the complexity of the current implementation. > This should significantly improve merge performance for iterators with > limited overlap (e.g. levelled compaction). -- This message was sent by Atlassian JIRA (v6.3.4#6332)