Engine condition pushdown is a MySQL internal mechanism that is intended to avoid to send non matching rows from the storage engine to the SQL layer. This is very important when the storage engine involves traffic over the network.
This mechanism was initially created to optimize MySQL Cluster (NDB) behavior.
For the NDB storage engine it avoids a lot of round trips through the network. Part of the filtering is done on the remote data node. Without engine condition pushed down the filtering is done by the sql layer after rows have been sent by the storage engine.
The MariaDB CONNECT Storage Engine implement various table type. Some of these table type correspond to remote access. This is the case of the ODBC and MySQL table type. These types can be considered as a more powerful evolution of the federated storage Engine.
In both case the engine condition pushdown gives a great benefit by avoiding network roundtrips.
The conditions are trapped by the storage engine handler through this API implementation:
const COND *ha_connect::cond_push(const COND *cond)
The "engine condition pushdown" mechanism should not be confused with the "index condition pushdown" optimization introduced in MariaDB 5.5 and MySQL 5.6. The index condition pushdown was introduced to use as much as possible the index to filter rows.