MySQL 5.7 Release Notes
The data exposed by the transaction and locking tables
(INNODB_TRX,
INNODB_LOCKS, and
INNODB_LOCK_WAITS) represents a
glimpse into fast-changing data. This is not like user tables,
where the data changes only 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
misleading joins between the transaction and locking 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 join 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_LOCKS.LOCK_DATA and
INNODB_TRX.TRX_QUERY), for security reasons,
only the users with the PROCESS privilege are
allowed to SELECT from them.
As described previously, the data that fills the transaction and
locking tables (INNODB_TRX,
INNODB_LOCKS and
INNODB_LOCK_WAITS) is fetched
automatically and saved to an intermediate buffer that provides
a “point-in-time” snapshot. The data across all
three tables is consistent when queried from the same snapshot.
However, the underlying data changes so fast that similar
glimpses at other, similarly fast-changing data, may not be in
synchrony. Thus, you should be careful when comparing data in
the InnoDB transaction and locking tables
with data in the PROCESSLIST table.
The data from the PROCESSLIST table
does not come from the same snapshot as the data about locking
and transactions. Even if you issue a single
SELECT (joining
INNODB_TRX and
PROCESSLIST, for example), the
content of those tables is generally not consistent.
INNODB_TRX may reference rows that
are not present in PROCESSLIST or
the currently executing SQL query of a transaction shown in
INNODB_TRX.TRX_QUERY may differ from the one
in PROCESSLIST.INFO.