zagto commented on code in PR #13333: URL: https://github.com/apache/arrow/pull/13333#discussion_r1083508144
########## docs/source/format/Columnar.rst: ########## @@ -765,6 +765,68 @@ application. We discuss dictionary encoding as it relates to serialization further below. +.. _run-length-encoded-layout: + +Run-Length Encoded Layout +------------------------- + +Run-Length is a data representation that represents data as sequences of the +same value, called runs. Each run is represented as a value, and an integer +describing how often this value is repeated. + +Any array can be run-length encoded. A run-length encoded array has no buffers +by itself, but has two child arrays. The first one holds a signed integer +called a "run end" for each run. The run ends array can hold either 16, 32, or +64-bit integers. The actual values of each run are held +the second child array. + +The values in the first child array represent the length of each run. They do +not hold the length of the respective run directly, but the accumulated length +of all runs from the first to the current one, i.e. the logical index where the +current run ends. This allows relatively efficient random access from a logical +index using binary search. The length of an individual run can be determined by +subtracting two adjacent values. + +A run must have have a length of at least 1. This means the values in the +run ends array all positive and in strictly ascending order. A run end cannot be +null. + +As an example, you could have the following data: :: + + type: Float32 + [1.0, 1.0, 1.0, 1.0, null, null, 2.0] + +In Run-length-encoded form, this could appear as: + +:: + + * Length: 7, Null count: 2 Review Comment: Yes, this is a general restriction (at least I think so, and that's how the code works). The idea so far is that Null is just one of the possible values for a run. The is somewhat consistent with how union types work. (if we were to allow the RLE array parent to have an additional null mask, the null count field would represent that - there seems to be a generall assumption in Arrow code that a non-zero (or array length for the NULL) null count means the presence of the standard null mask) -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: github-unsubscr...@arrow.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org
