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Encodings.md

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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.

GroupVarInt:

Supported Types: INT32, INT64

32-bit ints are encoded in groups of 4 with 1 leading bytes to encode the byte length of the following 4 ints.

64-bit are encoded in groups of 5, with 2 leading bytes to encode the byte length of the 5 ints.

For 32-bit ints, the leading byte contains 2 bits per int. Each length encoding specifies the number of bytes minus 1 for that int. For example a byte value of 0b00101101, indicates that:

  • the first int has 1 byte (0b00 + 1),
  • the second int has 3 bytes (0b10 + 1),
  • the third int has 4 bytes (0b11 + 1)
  • the 4th int has 2 bytes (0b01 + 1)

In this case, the entire row group would be: 1 + (1 + 3 + 4 + 2) = 11 bytes.
The bytes that follow the leading byte is just the int data encoded in little endian. With this example:

  • the first int starts at byte offset 1 with a max value of 0xFF,
  • the second int starts at byte offset 2 with a max value of 0xFFFFFF,
  • the third int starts at byte offset 5 with a max value of 0xFFFFFFFF, and
  • the 4th int starts at byte offset 9 with a max value of 0xFFFF.

For 64-bit ints, the lengths of the 5 ints is encoded as 3 bits. Combined, this uses 15 bits and fits in 2 bytes. The msb of the two bytes is unused. Like the 32-bit case, after the length bytes, the data bytes follow.

In the case where the data does not make a complete group, (e.g. 3 32-bit ints), a complete group should still be output with 0's filling in for the remainder. For example, if the input was (1,2,3,4,5): the resulting encoding should behave as if the input was (1,2,3,4,5,0,0,0) and the two groups should be encoded back to back.