Parquet encoding definitions

This file contains the specification of all supported encodings.

Plain:

Supported Types: all

This is the plain encoding that must be supported for types. It is intended to be the simplest encoding. Values are encoded back to back.

For native types, this outputs the data as little endian. Floating point types are encoded in IEEE.

For the byte array type, it encodes the length as a 4 byte little endian, followed by the bytes.

Delta Encoding

Supported Types: INT32, INT64

This encoding is adapted from the Binary packing described in “Decoding billions of integers per second through vectorization” by D. Lemire and L. Boytsov

Delta encoding consists of a header followed by blocks of delta encoded values binary packed. Each block is made of miniblocks each binary packed with its own bit width. When there are not enough values to encode a full block we pad with zeros (added to the frame of reference). The header contains:

<block size in values> <number of miniblocks in a block> <total value count> <first value>
  • the block size is a multiple of 128 stored as VLQ int
  • the miniblock count per block is a diviser of the block size stored as VLQ int the number of values in the miniblock is a multiple of 32.
  • the total value count is stored as a VLQ int
  • the first value is stored as a zigzag VLQ int

Each block contains

<min delta> <list of bitwidths of miniblocks> <miniblocks>
  • the min delta is a VLQ int (we compute a minimum as we need positive integers for bit packing)
  • the bitwidth of each block is stored as a byte
  • each miniblock is a list of bit packed ints according to the bit width stored at the begining of the block

Having multiple blocks allows us to escape values and restart from a new base value.

To encode each delta block, we will:

  1. Compute the deltas

  2. Encode the first value as zigzag VLQ int

  3. For each block, compute the frame of reference(minimum of the deltas) for the deltas. This guarantees all deltas are positive.

  4. encode the frame of reference delta as VLQ int followed by the delta values (minus the minimum) encoded as bit packed per miniblock.

Steps 2 and 3 are skipped if the number of values in the block is 1.

Example 1

1, 2, 3, 4, 5

After step 1), we compute the deltas as:

1, 1, 1, 1

The minimum delta is 1 and after step 2, the deltas become

0, 0, 0, 0

The final encoded data is:

header: 8 (block size), 1 (miniblock count), 5 (value count), 1 (first value)

block 1 (minimum delta), 0 (bitwidth), (no data needed for bitwidth 0)

Example 2

7, 5, 3, 1, 2, 3, 4, 5, the deltas would be

-2, -2, -2, 1, 1, 1, 1

The minimum is -2, so the relative deltas are:

0, 0, 0, 3, 3, 3, 3

The encoded data is

header: 8 (block size), 1 (miniblock count), 8 (value count), 7 (first value)

block 0 (minimum delta), 2 (bitwidth), 000000111111b (0,0,0,3,3,3 packed on 2 bits)

Delta-length byte array:

Supported Types: BYTE_ARRAY

This encoding is always preferred over PLAIN for byte array columns.

For this encoding, we will take all the byte array lengths and encode them using delta encoding (DELTA_BINARY_PACKED). The byte array data follows all of the length data just concatenated back to back. The expected savings is from the cost of encoding the lengths and possibly better compression in the data (it is no longer interleaved with the lengths).

The data stream looks like:

For example, if the data was “Hello”, “World”, “Foobar”, “ABCDEF”:

The encoded data would be DeltaEncoding(5, 5, 6, 6) “HelloWorldFoobarABCDEF”

Delta Strings:

Supported Types: BYTE_ARRAY

This is also known as incremental encoding or front compression: for each element in a sequence of strings, store the prefix length of the previous entry plus the suffix.

For a longer description, see http://en.wikipedia.org/wiki/Incremental_encoding.

This is stored as a sequence of delta-encoded prefix lengths (DELTA_BINARY_PACKED), followed by the suffixes encoded as delta length byte arrays (DELTA_LENGTH_BYTE_ARRAY).