The data exposed by the transaction and locking tables represent a glimpse into fast-changing data. This is not like other (user) tables, where the data only changes when application-initiated updates occur. The underlying data is internal system-managed data, and can change very quickly.
For performance reasons, and to minimize the chance of
JOINs between the
INFORMATION_SCHEMA tables, InnoDB collects
the required transaction and locking information into an
intermediate buffer whenever a
SELECT on any
of the tables is issued. This buffer is refreshed only if more
than 0.1 seconds has elapsed since the last time the buffer was
read. The data needed to fill the three tables is fetched
atomically and consistently and is saved in this global internal
buffer, forming a point-in-time “snapshot”. If
multiple table accesses occur within 0.1 seconds (as they almost
certainly do when MySQL processes a join among these tables),
then the same snapshot is used to satisfy the query.
A correct result is returned when you
any of these tables together in a single query, because the data
for the three tables comes from the same snapshot. Because the
buffer is not refreshed with every query of any of these tables,
if you issue separate queries against these tables within a
tenth of a second, the results are the same from query to query.
On the other hand, two separate queries of the same or different
tables issued more than a tenth of a second apart may see
different results, since the data come from different snapshots.
Because InnoDB must temporarily stall while the transaction and locking data is collected, too frequent queries of these tables can negatively impact performance as seen by other users.
As these tables contain sensitive information (at least
INNODB_TRX.TRX_QUERY), for security reasons,
only the users with the
PROCESS privilege are
SELECT from them.