Codec.Decompress

PQDriverless

Not part of the Power Query standard library. This function ships in PQDriverless, an open-source package of pure M readers for binary file formats — no ODBC driver, no OLE DB provider, nothing to install. Paste the M source into a blank query, or load PQDriverless.mez as a custom connector. See Installing.

Decompresses Snappy, Brotli, Zstandard, LZ4, GZip and Deflate streams — a drop-in superset of Binary.Decompress.

Syntax

Codec.Decompress(
    compressed as binary,
    compression as number,
    optional uncompressedSize as number,
    optional options as record
) as binary

Remarks

Binary.Decompress implements exactly two of the seven members of Compression.Type: GZip and Deflate. Ask it for Snappy, Brotli, LZ4 or Zstandard and it fails at runtime, even though the enum declares them.

The engine contains all of those codecs anyway — Parquet.Document uses them every time it reads a compressed Parquet file. Codec.Decompress makes them callable from plain M by wrapping an arbitrary compressed stream in a minimal, spec-conformant Parquet file and letting Parquet.Document do the work: one REQUIRED FIXED_LEN_BYTE_ARRAY column, one row, PLAIN encoding, which is the one point in Parquet's design space where the uncompressed data page is exactly the payload bytes. The compressed page can therefore be your stream, byte for byte, with no re-encoding.

GZip and Deflate route straight to Binary.Decompress; Compression.None returns the input unchanged.

The wrapper must declare the decompressed size, so the function reads it out of the stream itself:

Compression.Type	Wire format expected	Where the size comes from
Compression.Snappy	raw snappy block	preamble varint
Compression.Brotli	raw brotli stream	first meta-block MLEN
Compression.Zstandard	zstd frame	frame content size, when declared
Compression.LZ4	raw block or hadoop-framed, auto-detected	hadoop header, or a token walk over the sequences
Compression.GZip	gzip stream	not needed (native path)
Compression.Deflate	raw deflate	not needed (native path)
Compression.None	anything	returned unchanged

Two streams can genuinely withhold their size, and both raise a clear error asking for an explicit uncompressedSize: a zstd frame written without the optional content-size field, and a brotli stream the encoder split across several meta-blocks (typically inputs beyond a couple of MB). A wrong size can never corrupt the output — the Parquet reader checks it and fails loudly.

Options:

• LZ4Framing : Text, "Auto" (default), "Raw" or "Hadoop". Forces the LZ4 framing instead of detecting it from the stream.
• ReturnWrapper : A logical (true/false). Return the generated Parquet bytes instead of parsing them, for byte-level debugging (default is false).

Examples

Example #1

Decompress a snappy block.

Codec.Decompress(File.Contents("C:\data\block.snappy"), Compression.Snappy)

Example #2

Zstandard, where the frame doesn't declare its content size.

Codec.Decompress(blob, Compression.Zstandard, 1048576)

Example #3

Force a raw LZ4 framing rather than letting detection decide.

Codec.Decompress(blob, Compression.LZ4, null, [LZ4Framing = "Raw"])

Not supported

Framed container formats are not handled directly. A snappy framing-format file (sNaPpY magic) or an LZ4 frame is a sequence of blocks with chunk headers; split the frame with ordinary M byte work and feed each block through Codec.Decompress.

Excel is not one of the hosts. The whole technique rests on Parquet.Document, and Excel's Power Query does not implement it — the native Parquet libraries (ParquetSharp.dll and ParquetSharpNative.dll) ship with Power BI Desktop and are absent from Excel's Power Query add-in. So in Excel Codec.Decompress handles Compression.GZip, Compression.Deflate and Compression.None, which route to Binary.Decompress and never touch Parquet; Snappy, Brotli, Zstandard and LZ4 fail. See Where it runs.

Codec availability is otherwise a property of the host, not of this function. It has been verified in Power BI Desktop. The companion Codec.Probe function runs a fixture matrix against whichever host you point it at, so you can confirm Service refresh or Dataflows Gen2 for yourself before depending on it:

Codec.Probe("C:\path\to\powerquery-driverless\codec-oracle\test")

It takes the path of a folder of fixtures rather than a binary, because it compares every *.parquet fixture against payload.csv from the same folder. PQDriverless.mez exports the function but does not carry the fixtures — clone or download them from codec-oracle/test/ first, keeping the folder intact.

Category

Driverless connectors