Comparison performance
----------------------

LiveCompare is optimized for use on production systems and has various
parameters for tuning. Comparison rounds are read-only workloads. An
example use case compared 43,109,165 rows in 6 tables in 9m 17s with 4
connections and 4 workers, giving comparison performance of
approximately 77k rows per second, or 1 billion rows in <4 hours.

This use case is a general use case. For low-load, testing, migration,
and other specific scenarios, you might be able to improve speed by
changing the ``data_fetch_mode`` setting to use server-side cursors. In
our experiments, each kind of server-side cursors provides an increase
in performance on use cases involving either small or large tables.

Security considerations for the user
------------------------------------

For PostgreSQL 13 and earlier, LiveCompare requires a user that can read
all data being compared. PostgreSQL 14 introduced a new role,
pg_read_all_data, that can be used for LiveCompare.

When ``logical_replication_mode = bdr`` , LiveCompare requires a user
with the bdr_superuser role. When
``logical_replication_mode = pglogical`` , LiveCompare requires a user
with the ``pglogical_superuser`` role.

To apply the DML scripts in PGD, all divergent connections (potentially
all data connections) require a user with the bdr_superuser role to
disable ``bdr.xact_replication`` .

If PGD is being used, LiveCompare associates all fixed rows with a
replication origin called ``bdr_local_only_origin`` . LiveCompare also
applies the DML with the transaction datetime far in the past, so if
there are any PGD conflicts with real DML being executed on the
database, LiveCompare DML always loses the conflict.

With the default setting of ``difference_fix_start_query`` , the
transaction in apply scripts changes role to the owner of the table to
prevent database users from gaining access to the role applying fixes by
writing malicious triggers. As a result, the user for the divergent
connection needs to be able to switch role to the table owner.