The Performance Schema maintains statement digest information. Digesting converts a SQL statement to normalized form and computes a hash value for the result. Normalization permits statements that are similar to be grouped and summarized to expose information about the types of statements the server is executing and how often they occur. This section describes how statement normalizing occurs and how it can be useful.
Statement digesting involves these Performance Schema components:
statement_digest consumer in the
setup_consumers table controls
whether the Performance Schema maintains digest information.
The statement event tables
DIGEST_TEXT columns that contain digest MD5
values and the corresponding normalized statement text
table provides aggregated statement digest information.
Normalizing a statement transforms the statement text to a more standardized string representation that preserves the general statement structure while removing information not essential to the structure. Object identifiers such as database and table names are preserved. Values and comments are removed, and whitespace is adjusted. The Performance Schema does not retain information such as names, passwords, dates, and so forth.
Consider these statements:
SELECT * FROM orders WHERE customer_id=10 AND quantity>20 SELECT * FROM orders WHERE customer_id = 20 AND quantity > 100
To normalize these statements, the Performance Schema replaces
data values by
? and adjusts whitespace. Both
statements yield the same normalized form and thus are considered
SELECT * FROM orders WHERE customer_id = ? AND quantity > ?
The normalized statement contains less information but is still representative of the original statement. Other similar statements that have different comparison values have the same normalized form.
Now consider these statements:
SELECT * FROM customers WHERE customer_id = 1000 SELECT * FROM orders WHERE customer_id = 1000
In this case, the statements are not “the same.” The object identifiers differ, so the statements yield different normalized forms:
SELECT * FROM customers WHERE customer_id = ? SELECT * FROM orders WHERE customer_id = ?
Normalized statements have a fixed length. The maximum length of a
DIGEST_TEXT value is 1024 bytes. There is no
option to change this maximum. If normalization produces a
statement that exceeds this length, the text ends with
“...”. Long statements that differ only in the part
that occurs following the “...” are considered to be
the same. Consider these statements:
SELECT * FROM mytable WHERE cola = 10 AND colb = 20 SELECT * FROM mytable WHERE cola = 10 AND colc = 20
If the cutoff happened to be right after the
AND, both statements would have this normalized
SELECT * FROM mytable WHERE cola = ? AND ...
In this case, the difference in the second column name is lost and both statements are considered the same.
For each normalized statement, the Performance Schema computes a
hash digest value and stores that value and the statement in the
columns of the statement event tables
addition, information for statements with the same
values are aggregated in the
summary table. The Performance Schema uses MD5 hash values because
they are fast to compute and have a favorable statistical
distribution that minimizes collisions.
summary table has a fixed size, so when it becomes full,
statements that have
DIGEST values not matching existing values in
the table are grouped in a special row with
NULL. This permits all statements to be
counted. However, if the special row accounts for a significant
percentage of the statements executed, it might be desirable to
increase the size of the summary table. To do this, set the
system variable to a larger value at server startup. If no
value is given, the server estimates the value to use at startup.
The statement digest summary table provides a profile of the statements executed by the server. It shows what kinds of statements an application is executing and how often. An application developer can use this information together with other information in the table to assess the application's performance characteristics. For example, table columns that show wait times, lock times, or index use may highlight types of queries that are inefficient. This gives the developer insight into which parts of the application need attention.