Compressed Packet Header

Type	Name	Description
int<3>	length of compressed payload	raw packet length minus the size of the compressed packet header (7 bytes) itself.
int<1>	compressed sequence id	Sequence ID of the compressed packets, reset in the same way as the Protocol::Packet, but incremented independently
int<3>	length of uncompressed payload	Length of payload before compression

Compressed Payload

If the length of length of payload before compression is more than 0 the Compressed Packet Header is followed by the compressed payload.

Depending on which capability flags are set it uses the deflate algorithm as described in RFC 1951 and implemented in zlib or Zstandard.

When using zlib, the header of the compressed packet has the parameters of the uncompress() function in mind:

ZEXTERN int ZEXPORT uncompress OF((Bytef *dest, uLongf *destLen,

const Bytef *source, uLong sourceLen));

source

repeated Source source

Definition: replication_asynchronous_connection_failover.proto:42

The payload can be anything from a piece of a MySQL Packet to several MySQL Packets. The client or server may bundle several MySQL packets, compress it and send it as one compressed packet.

Example: One MySQL Packet

A COM_QUERY for select "012345678901234567890123456789012345" without CLIENT_COMPRESS has a payload length of 46 bytes and looks like:

2e 00 00 00 03 73 65 6c    65 63 74 20 22 30 31 32    .....select "012
34 35 36 37 38 39 30    31 32 33 34 35 36 37 38    3456789012345678
30 31 32 33 34 35 36    37 38 39 30 31 32 33 34    9012345678901234
22                                                 5"

With CLIENT_COMPRESS the packet is:

22 00 00 00 32 00 00 78    9c d3 63 60 60 60 2e 4e    "...2..x..c```.N
cd 49 4d 2e 51 50 32 30    34 32 36 31 35 33 b7 b0    .IM.QP20426153..
c4 cd 52 02 00 0c d1 0a    6c                         ..R.....l

comp-length	seq-id	uncomp-len	Compressed Payload
`22 00 00`	`00`	`32 00 00`	compress("\x2e\x00\x00\x00\x03select ...")`

The compressed packet is 41 bytes long and splits into:

raw packet length                      -> 41
compressed payload length   = 22 00 00 -> 34 (41 - 7)
sequence id                 = 00       ->  0
uncompressed payload length = 32 00 00 -> 50

Example: Several MySQL Packets

Executing SELECT repeat("a", 50) results in uncompressed ProtocolText::Resultset like:

00 00 01 01 25 00 00    02 03 64 65 66 00 00 00    .....%....def...
0f 72 65 70 65 61 74 28    22 61 22 2c 20 35 30 29    .repeat("a", 50)
0c 08 00 32 00 00 00    fd 01 00 1f 00 00 05 00    ....2...........
03 fe 00 00 02 00 33    00 00 04 32 61 61 61 61    .......3...2aaaa
61 61 61 61 61 61 61    61 61 61 61 61 61 61 61    aaaaaaaaaaaaaaaa
61 61 61 61 61 61 61    61 61 61 61 61 61 61 61    aaaaaaaaaaaaaaaa
61 61 61 61 61 61 61    61 61 61 61 61 61 05 00    aaaaaaaaaaaaaa..
05 fe 00 00 02 00                                  .......

which consists of 5 Protocol::Packet :

01 00 00 01 01
25 00 00 02 03 64 65 66 00 00 00 0f 72 65 70 65 61 74 28 22 61 22 2c 20 35 30 29 00 0c 08 00 32 00 00 00 fd 01 00 1f 00 00
05 00 00 03 fe 00 00 02 00
33 00 00 04 32 61 61 61 61 ...
05 00 00 05 fe 00 00 02 00

If compression is enabled a compressed packet containing the compressed version of all 5 packets is sent to the client:

4a 00 00 01 77 00 00 78    9c 63 64 60 60 64 54 65    J...w..x.cd``dTe
60 60 62 4e 49 4d 63 60    60 e0 2f 4a 2d 48 4d 2c    ``bNIMc``./J-HM,
d1 50 4a 54 d2 51 30 35    d0 64 e0 e1 60 30 02 8a    .PJT.Q05.d..`0..
ff 65 64 90 67 60 60 65    60 60 fe 07 54 cc 60 cc    .ed.g``e``..T.`.
c0 c0 62 94 48 32 00 ea    67 05 eb 07 00 8d f9 1c    ..b.H2..g.......
64                                                    d

Note: sending a MySQL Packet of the size 2²⁴-5 to 2²⁴-1 via compression leads to at least one extra compressed packet. If the uncompressed MySQL Packet is like
fe ff ff 03 ... -- length = 2^24-2, sequence id = 3

compressing it would result in the length of payload before compression in the Compressed Packet Header being:
length of mysql packet payload: 2^24-2

length of mysql packet header: 4

length of payload before compression: 2^24+2

mysql
Definition: instrumented_condition_variable.h:32

which can not be represented in one compressed packet. Instead two or more packets have to be sent.

Uncompressed Payload

For small packets it may be to costly to compress the packet:

compressing the packet may lead to more data and sending the data uncompressed
CPU overhead may be not worth to compress the data
Tip
Usually payloads less than 50 bytes (MIN_COMPRESS_LENGTH) aren't compressed.
To send an Uncompressed Payload :
set length of payload before compression in Compressed Packet Header to 0
The Compressed Payload contains the Uncompressed Payload instead.

Sending a SELECT 1 query as Uncompressed Payload to the server looks like:

0d 00 00 00 00 00 00 09 00 00 00 03 53 45 4c 45 ............SELE

43 54 20 31 CT 1

decodes into:

raw packet length                      -> 20
compressed payload length   = 0d 00 00 -> 13 (20 - 7)
sequence id                 = 00       ->  0
uncompressed payload length = 00 00 00 -> uncompressed

... with the uncompressed payload starting right after the 7 byte header:

09 00 00 00 03 53 45 4c 45 43 54 20 31 -- SELECT 1

consts::SELECT

const std::string SELECT("SELECT")

Name of the static privileges.