Details
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Improvement
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Status: Resolved
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Low
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Resolution: Fixed
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None
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None
Description
CollationController.collectAllData eliminates a sstable if we've already read a row tombstone more recent that its maxTimestamp. This is however done in 2 passes and can be inefficient (or rather, it's not as efficient as it could). More precisely, say we have 10 sstables s0, ... s9, where s0 is the most recent and s9 the least one (and their maxTimestamp reflect that) and s0 has a row tombstone that is more recent than all of s1-s9 maxTimestamps. Now in collectAllData(), we first iterate over sstables in a "random" order (because DataTracker keeps sstable in a more or less random order). Meaning that we may iterate in the order s9, s8, ... s0. In that case, we will end up reading the row header from all the sstable (hitting disk each time). Then, and only then, the 2nd pass of collectAllData will eliminate s1 to s9.
However, if we were to iterate sstable in maxTimestamps order (as we do in collectTimeOrdered), we would only need one pass but more importantly we would minimize the number of row header we read to perform that sstable eliminination. In my example, we would only ever read the row tombstone from s0 and eliminate all other sstable directly, simply based on their maxTimestamp.