As of MySQL 5.6.5, the MySQL server is capable of maintaining statement digest information. The digesting process converts an 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.

In the Performance Schema, statement digesting involves these components:

Statement normalization transforms the statement text to a more standardized digest string representation that preserves the general statement structure while removing information not essential to the structure:

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 “the same”:

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 = ?

If normalization produces a statement that exceeds the space available in the digest buffer, 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 happens to be right after the AND, both statements have this normalized form:

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 the statement and its MD5 hash value in the DIGEST_TEXT and DIGEST columns of the statement event tables (events_statements_current, events_statements_history, and events_statements_history_long). In addition, statement digests are summarized in the events_statements_summary_by_digest table, which aggregates information for statements that have the same SCHEMA_NAME and DIGEST values. The Performance Schema uses MD5 hash values because they are fast to compute and have a favorable statistical distribution that minimizes collisions.

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.

The events_statements_summary_by_digest summary table has a fixed size. When it becomes full, statements that have SCHEMA_NAME and DIGEST values not matching existing values in the table are grouped in a special row with SCHEMA_NAME and DIGEST set to 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 by setting the performance_schema_digests_size system variable to a larger value at server startup. If no performance_schema_digests_size value is given, the server estimates the value to use at startup. (Before MySQL 5.6.9, there is no SCHEMA_NAME column and the special row has DIGEST set to NULL.)

The performance_schema_max_digest_length system variable determines the maximum number of bytes available in the digest buffer for digest computation. However, the display length of statement digests may be longer than the available buffer size due to internal encoding of statement components such as keywords and literal values. Consequently, values selected from the DIGEST_TEXT column of statement event tables may appear to exceed the performance_schema_max_digest_length value.

For applications that generate very long statements that differ only at the end, increasing performance_schema_max_digest_length enables computation of digests that distinguish statements that would otherwise aggregate to the same digest. Conversely, decreasing performance_schema_max_digest_length causes the server to devote less memory to digest storage but increases the likelihood of longer statements aggregating to the same digest. Administrators should keep in mind that larger values result in correspondingly increased memory requirements, particularly for workloads that involve large numbers of simultaneous sessions (performance_schema_max_digest_length bytes are allocated per session).