Connections to the server can use a compressed protocol that reduces the number of bytes sent over the connection. By default, connections are uncompressed, but can be compressed if the server and the client both support compression.

Compressed connections originate on the client side but affect CPU load on both the client and server sides because both sides perform compression and decompression operations. Because enabling compression decreases performance, its benefits occur primarily when there is low network bandwidth, network transfer time dominates the cost of compression and decompression operations, and result sets are large.

Compression control applies to connections to the server by client programs and by servers participating in master/slave replication. Compression control does not apply to Group Replication connections, X Protocol connections, or connections for FEDERATED tables.

These configuration parameters are available for controlling connection compression:

In each case, when use of compression is specified, the connection uses the zlib compression algorithm if both sides support it, with fallback to an uncompressed connection otherwise.