SET SINGLE_SOURCE_FEDERATION#
Controls whether Querona may push a federated query down to the single source that already holds
most of the data. When ON (the default) and a query joins a large table on one source with a
small table on another, Querona can run the whole query on the source with the large table, avoiding
the transfer of a large result to a federator. When OFF, this optimization is skipped.
Tip
This is an advanced query-planning option, on by default. It affects only how a federated query is planned and executed, not its results — change it for diagnostics or to work around a specific federation issue.
Syntax#
SET SINGLE_SOURCE_FEDERATION { ON | OFF } [;]
Remarks#
Moving a large result set to a federator is expensive. When most of a federated query’s data lives on
one source that can also act as a federator, running the whole query there is usually cheaper. This
option enables that arrangement; it relies on cardinality estimates to decide which source holds the
larger share of data. The default is ON.
See SET EXTERNAL_FEDERATION and SET IN_MEMORY_FEDERATION for the other federation controls.
Permissions#
Requires membership in the public role.
Examples#
SET SINGLE_SOURCE_FEDERATION OFF;