4f865f7870
After inserts go into a compressed chunk, the chunk is marked as unordered.This PR adds a new function recompress_chunk that compresses the data and sets the status back to compressed. Further optimizations for this function are planned but not part of this PR. This function can be invoked by calling SELECT recompress_chunk(<chunk_name>). recompress_chunk function is automatically invoked by the compression policy job, when it sees that a chunk is in unordered state.
Compression Algorithms
This is a collection of compression algorithms that are used to compress data of different types. The algorithms are optimized for time-series use-cases; many of them assume that adjacent rows will have "similar" values.
API
Each compression algorithm the API is divided into two parts: a compressor and a decompression iterator. The compressor is used to compress new data.
<algorithm name>_compressor_alloc- creates the compressor<algorithm_name>_compressor_append_null- appends a null<algorithm_name>_compressor_append_value- appends a non-null value<agorithm_name>_compressor_finish- finalizes the compression and returns the compressed data
Data can be read back out using the decompression iterator. An iterator can operate backwards or forwards. There is no random access. The api is
<algorithm_name>_decompression_iterator_from_datum_<forward|reverse>- create a new DatumIterator in the forward or reverse direction.- a DatumIterator has a function pointer called
try_nextthat returns the nextDecompressResult.
A DecompressResult can either be a decompressed value datum, null, or a done marker to indicate that the iterator is done.
Each decompression algorithm also contains send and recv function to get the external binary representations.
CompressionAlgorithmDefinition is a structure that defines function pointers to get forward and reverse iterators
as well as send and recv functions. The definitions array in compression.c contains a CompressionAlgorithmDefinition
for each compression algorithm.
Base algorithms
The simple8b rle algorithm is a building block for many of the compression algorithms.
It compresses a series of uint64 values. It compresses the data by packing the values into the least
amount of bits necessary for the magnitude of the int values, using run-length-encoding for large numbers of repeated values,
A complete description is in the header file. Note that this is a header-only implementation as performance
is paramount here as it is used a primitive in all the other compression algorithms.
Compression Algorithms
DeltaDelta
for each integer, it takes the delta-of-deltas with the pervious integer, zigzag encodes this deltadelta, then finally simple8b_rle encodes this zigzagged result. This algorithm performs very well when the magnitude of the delta between adjacent values tends not to vary much, and is optimal for fixed rate-of-change.
Gorilla
gorilla encodes floats using the Facebook gorilla algorithm. It stores the
compressed xors of adjacent values. It is one of the few simple algorithms
that compresses floating point numbers reasonably well.
Dictionary
The dictionary mechanism stores data in two parts: a "dictionary" storing each unique value in the dataset (stored as an array, see below) and simple8b_rle compressed list of indexes into the dictionary, ordered by row. This scheme can store any type of data, but will only be a space improvement if the data set is of relatively low cardinality.
Array
The array "compression" method simply stores the data in an array-like structure and does not actually compress it (though TOAST-based compression can be applied on top). It is the compression mechanism used when no other compression mechanism works. It can store any type of data.