当解析器接收到 SQL 语句时，如果需要该摘要，则会计算语句摘要。如果满足以下任一条件，则需要计算摘要：When the parser receives an SQL statement, it computes a statement digest if that digest is needed, which is true if any of the following conditions are true:

解析器还被 STATEMENT_DIGEST_TEXT() 和 STATEMENT_DIGEST() 函数使用，应用程序可以调用这些函数分别从 SQL 语句计算标准化语句摘要和摘要哈希值。The parser is also used by the STATEMENT_DIGEST_TEXT() and STATEMENT_DIGEST() functions, which applications can call to compute a normalized statement digest and a digest hash value, respectively, from an SQL statement.

max_digest_length 系统变量的值决定了每个会话可用于计算规范化语句摘要的最大字节数。一旦在摘要计算过程中使用了该空间，就会发生截断：不会再收集解析语句的其他标记，也不会将其纳入摘要值。仅在解析标记的字节数超过该限制后才不同的语句会生成相同的规范化语句摘要，如果进行比较或聚合摘要统计，它们会被视为相同。The max_digest_length system variable value determines the maximum number of bytes available per session for computation of normalized statement digests. Once that amount of space is used during digest computation, truncation occurs: no further tokens from a parsed statement are collected or figure into its digest value. Statements that differ only after that many bytes of parsed tokens produce the same normalized statement digest and are considered identical if compared or if aggregated for digest statistics.

在计算出标准化语句之后，会从中计算出 SHA-256 哈希值。此外：After the normalized statement has been computed, a SHA-256 hash value is computed from it. In addition:

语句规范化将语句文本转换为更标准的摘要字符串表示形式，同时保留一般语句结构，而去除对结构不必要的信息：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:

考虑以下语句：Consider these statements:

SELECT * FROM orders WHERE customer_id=10 AND quantity>20
SELECT * FROM orders WHERE customer_id = 20 AND quantity > 100

为了规范化这些语句，解析器将数据值替换为?并调整空白。两个语句生成相同的规范化形式，因此被认为是“相同”：To normalize these statements, the parser 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 > ?

归一化语句包含的信息较少，但仍能代表原始语句。其他具有不同数据值的类似语句具有相同的归一化形式。The normalized statement contains less information but is still representative of the original statement. Other similar statements that have different data values have the same normalized form.

现在考虑这些语句：Now consider these statements:

SELECT * FROM customers WHERE customer_id = 1000
SELECT * FROM orders WHERE customer_id = 1000

在这种情况下，归一化语句不同，因为对象标识符不同：In this case, the normalized statements differ because the object identifiers differ:

SELECT * FROM customers WHERE customer_id = ?
SELECT * FROM orders WHERE customer_id = ?

如果归一化产生的语句超过了摘要缓冲区可用的空间（由 max_digest_length 决定），则会发生截断，并且文本以 “...” 结尾。仅在 “...” 后部分不同的长归一化语句被视为相同。考虑这些语句：If normalization produces a statement that exceeds the space available in the digest buffer (as determined by max_digest_length), truncation occurs and the text ends with “...”. Long normalized statements that differ only in the part that occurs following the “...” are considered the same. Consider these statements:

SELECT * FROM mytable WHERE cola = 10 AND colb = 20
SELECT * FROM mytable WHERE cola = 10 AND colc = 20

如果截断恰好发生在AND之后，两条语句都有这种规范化形式：If the cutoff happens to be right after the AND, both statements have this normalized form:

SELECT * FROM mytable WHERE cola = ? AND ...

在这种情况下，第二列名称的差异会丢失，两条语句被认为是相同的。In this case, the difference in the second column name is lost and both statements are considered the same.

在性能模式中，语句摘要涉及以下元素：In the Performance Schema, statement digesting involves these elements:

一些性能表有一列存储用于计算摘要的原始 SQL 语句：Some Performance Tables have a column that stores original SQL statements from which digests are computed:

默认情况下，语句显示的最大可用空间为 1024 字节。要更改此值，请在服务器启动时设置performance_schema_max_sql_text_length系统变量。更改会影响刚才列出的所有列所需的存储空间。The maximum space available for statement display is 1024 bytes by default. To change this value, set the performance_schema_max_sql_text_length system variable at server startup. Changes affect the storage required for all the columns just named.

performance_schema_max_digest_length 系统变量决定 Performance Schema 中每个语句用于存储摘要值的最大字节数。然而，由于语句元素（如关键字和字面值）的内部编码，语句摘要的显示长度可能会超过可用的缓冲区大小。因此，从语句事件表的 DIGEST_TEXT 列中选择的值可能看起来超过 performance_schema_max_digest_length 值。The performance_schema_max_digest_length system variable determines the maximum number of bytes available per statement for digest value storage in the Performance Schema. However, the display length of statement digests may be longer than the available buffer size due to internal encoding of statement elements 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.

events_statements_summary_by_digest 汇总表提供了服务器执行语句的概况。它显示了应用程序正在执行的语句类型以及执行的频率。应用程序开发者可以将这些信息与表中的其他信息结合起来评估应用程序的性能特征。例如，显示等待时间、锁定时间或索引使用情况的表列可能会突出显示效率低下的查询类型。这使开发者能够了解应用程序哪些部分需要关注。The events_statements_summary_by_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.

events_statements_summary_by_digest 汇总表的大小是固定的。默认情况下，Performance Schema 会在启动时估算要使用的大小。要显式指定表的大小，请在服务器启动时设置 performance_schema_digests_size 系统变量。如果表已满，Performance Schema 会将 SCHEMA_NAME 和 DIGEST 值与表中现有值不匹配的语句分组到一个特殊行中，该行的 SCHEMA_NAME 和 DIGEST 设置为 NULL。这允许统计所有语句的数量。然而，如果该特殊行占执行语句的显著百分比，可能需要通过增加 performance_schema_digests_size 来增大汇总表的大小。The events_statements_summary_by_digest summary table has a fixed size. By default the Performance Schema estimates the size to use at startup. To specify the table size explicitly, set the performance_schema_digests_size system variable at server startup. If the table becomes full, the Performance Schema groups statements that have SCHEMA_NAME and DIGEST values not matching existing values in the table 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 summary table size by increasing performance_schema_digests_size.

对于生成非常长且仅在末尾不同的语句的应用程序，增加max_digest_length可以计算出区分那些原本会聚合到相同摘要的语句的摘要。相反，减少max_digest_length会使服务器在摘要存储上分配更少的内存，但会增加较长语句聚合到相同摘要的可能性。管理员应记住，更大的值会相应地增加内存需求，尤其是对于涉及大量并发会话的工作负载（服务器为每个会话分配max_digest_length字节）。For applications that generate very long statements that differ only at the end, increasing max_digest_length enables computation of digests that distinguish statements that would otherwise aggregate to the same digest. Conversely, decreasing 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 (the server allocates max_digest_length bytes per session).

如前所述，由解析器计算的规范化语句摘要限制为最大 max_digest_length 字节，而存储在性能模式中的规范化语句摘要使用 performance_schema_max_digest_length 字节。关于 max_digest_length 和 performance_schema_max_digest_length 相对值的内存使用考虑如下：As described previously, normalized statement digests as computed by the parser are constrained to a maximum of max_digest_length bytes, whereas normalized statement digests stored in the Performance Schema use performance_schema_max_digest_length bytes. The following memory-use considerations apply regarding the relative values of max_digest_length and performance_schema_max_digest_length:

因为 Performance Schema 语句事件表可能存储许多摘要，将 performance_schema_max_digest_length 设置得比 max_digest_length 小，使管理员能够在以下因素之间取得平衡：Because the Performance Schema statement event tables might store many digests, setting performance_schema_max_digest_length smaller than max_digest_length enables administrators to balance these factors:

performance_schema_max_digest_length 设置不是按会话的，它是按语句的，并且一个会话可以在 events_statements_history 表中存储多个语句。该表中每个会话的语句数量通常为 10，因此仅该表，每个会话消耗的内存是 performance_schema_max_digest_length 值的 10 倍。The performance_schema_max_digest_length setting is not per session, it is per statement, and a session can store multiple statements in the events_statements_history table. A typical number of statements in this table is 10 per session, so each session consumes 10 times the memory indicated by the performance_schema_max_digest_length value, for this table alone.

此外，全球还收集了许多语句（和摘要），最著名的是 events_statements_history_long 表。在这里，存储 N 条语句会消耗 N 倍于 performance_schema_max_digest_length 值所示的内存。Also, there are many statements (and digests) collected globally, most notably in the events_statements_history_long table. Here, too, N statements stored consumes N times the memory indicated by the performance_schema_max_digest_length value.

要评估用于 SQL 语句存储和摘要计算的内存量，请使用 SHOW ENGINE PERFORMANCE_SCHEMA STATUS 语句，或监控以下工具：To assess the amount of memory used for SQL statement storage and digest computation, use the SHOW ENGINE PERFORMANCE_SCHEMA STATUS statement, or monitor these instruments:

mysql> SELECT NAME
       FROM performance_schema.setup_instruments
       WHERE NAME LIKE '%.sqltext';
+------------------------------------------------------------------+
| NAME                                                             |
+------------------------------------------------------------------+
| memory/performance_schema/events_statements_history.sqltext      |
| memory/performance_schema/events_statements_current.sqltext      |
| memory/performance_schema/events_statements_history_long.sqltext |
+------------------------------------------------------------------+

mysql> SELECT NAME
       FROM performance_schema.setup_instruments
       WHERE NAME LIKE 'memory/performance_schema/%.tokens';
+----------------------------------------------------------------------+
| NAME                                                                 |
+----------------------------------------------------------------------+
| memory/performance_schema/events_statements_history.tokens           |
| memory/performance_schema/events_statements_current.tokens           |
| memory/performance_schema/events_statements_summary_by_digest.tokens |
| memory/performance_schema/events_statements_history_long.tokens      |
+----------------------------------------------------------------------+

Performance Schema 使用语句抽样来收集在 events_statements_summary_by_digest 表中生成每个摘要值的代表性语句。这些列存储样本语句信息：QUERY_SAMPLE_TEXT（语句文本）、QUERY_SAMPLE_SEEN（语句被查看的时间）和 QUERY_SAMPLE_TIMER_WAIT（语句等待或执行时间）。每次选择样本语句时，Performance Schema 都会更新这三列。The Performance Schema uses statement sampling to collect representative statements that produce each digest value in the events_statements_summary_by_digest table. These columns store sample statement information: QUERY_SAMPLE_TEXT (the text of the statement), QUERY_SAMPLE_SEEN (when the statement was seen), and QUERY_SAMPLE_TIMER_WAIT (the statement wait or execution time). The Performance Schema updates all three columns each time it chooses a sample statement.

当插入新的表格行时，生成该行摘要值的语句会被存储为与摘要关联的当前示例语句。此后，当服务器遇到具有相同摘要值的其他语句时，它会确定是否使用新语句来替换当前示例语句（即是否重新采样）。重新采样策略基于当前示例语句和新语句的等待时间比较，并且可选地考虑当前示例语句的年龄：When a new table row is inserted, the statement that produced the row digest value is stored as the current sample statement associated with the digest. Thereafter, when the server sees other statements with the same digest value, it determines whether to use the new statement to replace the current sample statement (that is, whether to resample). Resampling policy is based on the comparative wait times of the current sample statement and new statement and, optionally, the age of the current sample statement:

默认情况下，performance_schema_max_digest_sample_age 为 60 秒（1 分钟）。要更改样本语句因年龄而“过期”的速度，请增加或减少该值。要禁用基于年龄的重新采样策略部分，请将 performance_schema_max_digest_sample_age 设置为 0。By default, performance_schema_max_digest_sample_age is 60 seconds (1 minute). To change how quickly sample statements “expire” due to age, increase or decrease the value. To disable the age-based part of the resampling policy, set performance_schema_max_digest_sample_age to 0.