关于 PreparedStatement

news2024/9/20 11:01:26

Mysql 层面的语法也支持 prepare
这个确实第一次见

  • PREPARE prepares a statement for execution (see Section 13.5.1, “PREPARE Statement”).
  • EXECUTE executes a prepared statement (see Section 13.5.2, “EXECUTE Statement”).
  • DEALLOCATE PREPARE releases a prepared statement (see Section 13.5.3, “DEALLOCATE PREPARE Statement”).
mysql> PREPARE stmt1 FROM 'SELECT * FROM words where id = ?';
Query OK, 0 rows affected (0.00 sec)
Statement prepared

mysql> SET @i=1;
Query OK, 0 rows affected (0.00 sec)
mysql> EXECUTE stmt1 USING @i;
Empty set (0.01 sec)

mysql> SET @i=2;
Query OK, 0 rows affected (0.00 sec)
mysql> EXECUTE stmt1 USING @i;
+----+------+
| id | word |
+----+------+
|  2 | 123  |
+----+------+
1 row in set (0.00 sec)

mysql> deallocate prepare stmt1;
Query OK, 0 rows affected (0.00 sec)

mysql> EXECUTE stmt1 USING @i;
ERROR 1243 (HY000): Unknown prepared statement handler (stmt1) given to EXECUTE

useServerPrepStmts=true

useServerPrepStmts

Use server-side prepared statements if the server supports them?

Default: false

Since version: 3.1.0

如果我们没有在 jdbc 参数中声明这个参数、即使我们在代码中使用了 PreparedStatement 实际上是毫无作用的

    try (PreparedStatement psts = conn.prepareStatement("delete from words where id = ?")) {
      psts.setInt(1, 1);
      psts.execute();
      stopwatch.stop();
      System.out.println("stopwatch = " + stopwatch.elapsed(TimeUnit.MILLISECONDS));
      psts.setInt(1, 10);
      psts.execute();
    }

抓包看看

在这里插入图片描述

mysql general log:

2020-05-04T13:06:07.883131Z	   15 Connect	root@localhost on test using TCP/IP
2020-05-04T13:06:07.885668Z	   15 Query	/* mysql-connector-java-8.0.16 (Revision: 34cbc6bc61f72836e26327537a432d6db7c77de6) */SELECT  @@session.auto_increment_increment AS auto_increment_increment, @@character_set_client AS character_set_client, @@character_set_connection AS character_set_connection, @@character_set_results AS character_set_results, @@character_set_server AS character_set_server, @@collation_server AS collation_server, @@collation_connection AS collation_connection, @@init_connect AS init_connect, @@interactive_timeout AS interactive_timeout, @@license AS license, @@lower_case_table_names AS lower_case_table_names, @@max_allowed_packet AS max_allowed_packet, @@net_write_timeout AS net_write_timeout, @@performance_schema AS performance_schema, @@sql_mode AS sql_mode, @@system_time_zone AS system_time_zone, @@time_zone AS time_zone, @@transaction_isolation AS transaction_isolation, @@wait_timeout AS wait_timeout
2020-05-04T13:06:07.905021Z	   15 Query	SET character_set_results = NULL
2020-05-04T13:06:07.929557Z	   15 Query	delete from words where id = 1
2020-05-04T13:06:07.934906Z	   15 Query	delete from words where id = 10
2020-05-04T13:06:07.940645Z	   15 Quit

当我们使用 useServerPrepStmts=true 之后

在这里插入图片描述在这里插入图片描述
在这里插入图片描述

log

2020-05-04T13:15:55.611149Z	   16 Connect	root@localhost on test using TCP/IP
2020-05-04T13:15:55.615655Z	   16 Query	/* mysql-connector-java-8.0.16 (Revision: 34cbc6bc61f72836e26327537a432d6db7c77de6) */SELECT  @@session.auto_increment_increment AS auto_increment_increment, @@character_set_client AS character_set_client, @@character_set_connection AS character_set_connection, @@character_set_results AS character_set_results, @@character_set_server AS character_set_server, @@collation_server AS collation_server, @@collation_connection AS collation_connection, @@init_connect AS init_connect, @@interactive_timeout AS interactive_timeout, @@license AS license, @@lower_case_table_names AS lower_case_table_names, @@max_allowed_packet AS max_allowed_packet, @@net_write_timeout AS net_write_timeout, @@performance_schema AS performance_schema, @@sql_mode AS sql_mode, @@system_time_zone AS system_time_zone, @@time_zone AS time_zone, @@transaction_isolation AS transaction_isolation, @@wait_timeout AS wait_timeout
2020-05-04T13:15:55.640086Z	   16 Query	SET character_set_results = NULL
2020-05-04T13:15:55.672345Z	   16 Prepare	delete from words where id = ?
2020-05-04T13:15:55.676355Z	   16 Execute	delete from words where id = 1
2020-05-04T13:15:55.681600Z	   16 Execute	delete from words where id = 10
2020-05-04T13:15:55.681949Z	   16 Close stmt
2020-05-04T13:15:55.687659Z	   16 Quit

cachePrepStmts

客户端 connection 级别缓存预编译的 sql

@Test
@SneakyThrows
public void testPreCompile() {
  String connectString = "jdbc:mysql://localhost/test?user=root&password=toor&useLocalSessionState=true&useSSL=false&useServerPrepStmts=true&cachePrepStmts=true";
  Class.forName("com.mysql.jdbc.Driver")
    .newInstance();
  try (Connection conn = DriverManager.getConnection(connectString)) {
    Stopwatch stopwatch = Stopwatch.createStarted();
    try (PreparedStatement psts = conn.prepareStatement("delete from words where id = ?")) {
      psts.setInt(1, 1);
      psts.execute();
      stopwatch.stop();
      System.out.println("stopwatch = " + stopwatch.elapsed(TimeUnit.MILLISECONDS));
      psts.setInt(1, 10);
      psts.execute();
    }

    // 上面的stmt关闭之后，再次执行
    try (PreparedStatement psts = conn.prepareStatement("delete from words where id = ?")) {
      psts.setInt(1, 100);
      psts.execute();
    }
  }

  // 上面的connection关闭之后，再次执行
  try (Connection conn = DriverManager.getConnection(connectString)) {
    try (PreparedStatement psts = conn.prepareStatement("delete from words where id = ?")) {
      psts.setInt(1, 66);
      psts.execute();
    }
  }

}

在这里插入图片描述

2020-05-04T15:17:32.921041Z	   19 Connect	root@localhost on test using TCP/IP
2020-05-04T15:17:32.929561Z	   19 Query	/* mysql-connector-java-8.0.16 (Revision: 34cbc6bc61f72836e26327537a432d6db7c77de6) */SELECT  @@session.auto_increment_increment AS auto_increment_increment, @@character_set_client AS character_set_client, @@character_set_connection AS character_set_connection, @@character_set_results AS character_set_results, @@character_set_server AS character_set_server, @@collation_server AS collation_server, @@collation_connection AS collation_connection, @@init_connect AS init_connect, @@interactive_timeout AS interactive_timeout, @@license AS license, @@lower_case_table_names AS lower_case_table_names, @@max_allowed_packet AS max_allowed_packet, @@net_write_timeout AS net_write_timeout, @@performance_schema AS performance_schema, @@sql_mode AS sql_mode, @@system_time_zone AS system_time_zone, @@time_zone AS time_zone, @@transaction_isolation AS transaction_isolation, @@wait_timeout AS wait_timeout
2020-05-04T15:17:32.949986Z	   19 Query	SET character_set_results = NULL
2020-05-04T15:17:32.983173Z	   19 Prepare	delete from words where id = ?
2020-05-04T15:17:32.990498Z	   19 Execute	delete from words where id = 1
2020-05-04T15:17:32.997115Z	   19 Execute	delete from words where id = 10
2020-05-04T15:17:32.997566Z	   19 Reset stmt
2020-05-04T15:17:32.997725Z	   19 Execute	delete from words where id = 100
2020-05-04T15:17:33.003682Z	   19 Quit
2020-05-04T15:17:33.009206Z	   20 Connect	root@localhost on test using TCP/IP
2020-05-04T15:17:33.009643Z	   20 Query	/* mysql-connector-java-8.0.16 (Revision: 34cbc6bc61f72836e26327537a432d6db7c77de6) */SELECT  @@session.auto_increment_increment AS auto_increment_increment, @@character_set_client AS character_set_client, @@character_set_connection AS character_set_connection, @@character_set_results AS character_set_results, @@character_set_server AS character_set_server, @@collation_server AS collation_server, @@collation_connection AS collation_connection, @@init_connect AS init_connect, @@interactive_timeout AS interactive_timeout, @@license AS license, @@lower_case_table_names AS lower_case_table_names, @@max_allowed_packet AS max_allowed_packet, @@net_write_timeout AS net_write_timeout, @@performance_schema AS performance_schema, @@sql_mode AS sql_mode, @@system_time_zone AS system_time_zone, @@time_zone AS time_zone, @@transaction_isolation AS transaction_isolation, @@wait_timeout AS wait_timeout
2020-05-04T15:17:33.010569Z	   20 Query	SET character_set_results = NULL
2020-05-04T15:17:33.011244Z	   20 Prepare	delete from words where id = ?
2020-05-04T15:17:33.011475Z	   20 Execute	delete from words where id = 66
2020-05-04T15:17:33.114892Z	   20 Quit

我们直接来看看它是怎么存储的

在这里插入图片描述

在这里插入图片描述

既然是缓存、肯定涉及到缓存的个数以及单个缓存值的大小

prepStmtCacheSize & prepStmtCacheSqlLimit

prepStmtCacheSize 默认值为 25 注意这里是一个 connection 下最多缓存 25 个预编译的 Statement

prepStmtCacheSqlLimit 默认是 256 单纯就是 sql 的长度

在这里插入图片描述

源码

在这里插入图片描述

com.mysql.cj.jdbc.ConnectionImpl#prepareStatement(java.lang.String, int, int)

    @Override
    public java.sql.PreparedStatement prepareStatement(String sql, int resultSetType, int resultSetConcurrency) throws SQLException {
        synchronized (getConnectionMutex()) {
            checkClosed();

            //
            // FIXME: Create warnings if can't create results of the given type or concurrency
            //
            ClientPreparedStatement pStmt = null;

            boolean canServerPrepare = true;

            String nativeSql = this.processEscapeCodesForPrepStmts.getValue() ? nativeSQL(sql) : sql;
			// useServerPrepStmts 属性
            if (this.useServerPrepStmts.getValue() && this.emulateUnsupportedPstmts.getValue()) {
                canServerPrepare = canHandleAsServerPreparedStatement(nativeSql);
            }

            if (this.useServerPrepStmts.getValue() && canServerPrepare) {
				// cachePrepStmts 缓存预编译 Statement
                if (this.cachePrepStmts.getValue()) {
                    synchronized (this.serverSideStatementCache) {
						// 从缓存中获取
                        pStmt = this.serverSideStatementCache.remove(new CompoundCacheKey(this.database, sql));

                        if (pStmt != null) {
							// 强转为ServerPreparedStatement，清理参数，直接返回
                            ((com.mysql.cj.jdbc.ServerPreparedStatement) pStmt).setClosed(false);
                            pStmt.clearParameters();
                        }

                        if (pStmt == null) {
                            try {
                                pStmt = ServerPreparedStatement.getInstance(getMultiHostSafeProxy(), nativeSql, this.database, resultSetType,
                                        resultSetConcurrency);
                                if (sql.length() < this.prepStmtCacheSqlLimit.getValue()) {
                                    ((com.mysql.cj.jdbc.ServerPreparedStatement) pStmt).isCacheable = true;
                                }

                                pStmt.setResultSetType(resultSetType);
                                pStmt.setResultSetConcurrency(resultSetConcurrency);
                            } catch (SQLException sqlEx) {
                                // Punt, if necessary
                                if (this.emulateUnsupportedPstmts.getValue()) {
                                    pStmt = (ClientPreparedStatement) clientPrepareStatement(nativeSql, resultSetType, resultSetConcurrency, false);

                                    if (sql.length() < this.prepStmtCacheSqlLimit.getValue()) {
                                        this.serverSideStatementCheckCache.put(sql, Boolean.FALSE);
                                    }
                                } else {
                                    throw sqlEx;
                                }
                            }
                        }
                    }
                } else {
                    try {
                        pStmt = ServerPreparedStatement.getInstance(getMultiHostSafeProxy(), nativeSql, this.database, resultSetType, resultSetConcurrency);

                        pStmt.setResultSetType(resultSetType);
                        pStmt.setResultSetConcurrency(resultSetConcurrency);
                    } catch (SQLException sqlEx) {
                        // Punt, if necessary
                        if (this.emulateUnsupportedPstmts.getValue()) {
                            pStmt = (ClientPreparedStatement) clientPrepareStatement(nativeSql, resultSetType, resultSetConcurrency, false);
                        } else {
                            throw sqlEx;
                        }
                    }
                }
            } else {
                pStmt = (ClientPreparedStatement) clientPrepareStatement(nativeSql, resultSetType, resultSetConcurrency, false);
            }

            return pStmt;
        }
    }

close 的时候会重新放回去缓存

// com.mysql.cj.jdbc.ServerPreparedStatement#close
 @Override
public void close() throws SQLException {
  JdbcConnection locallyScopedConn = this.connection;

  if (locallyScopedConn == null) {
    return; // already closed
  }

  synchronized (locallyScopedConn.getConnectionMutex()) {

    if (this.isCached && isPoolable() && !this.isClosed) {
      clearParameters();

      this.isClosed = true;
			// 重新缓存起来
      this.connection.recachePreparedStatement(this);
      return;
    }

    this.isClosed = false;
    realClose(true, true);
  }
}

// com.mysql.cj.jdbc.ConnectionImpl#recachePreparedStatement
    @Override
    public void recachePreparedStatement(JdbcPreparedStatement pstmt) throws SQLException {
        synchronized (getConnectionMutex()) {
            if (this.cachePrepStmts.getValue() && pstmt.isPoolable()) {
                synchronized (this.serverSideStatementCache) {
                    Object oldServerPrepStmt = this.serverSideStatementCache.put(
                            new CompoundCacheKey(pstmt.getCurrentCatalog(), ((PreparedQuery<?>) pstmt.getQuery()).getOriginalSql()),
                            (ServerPreparedStatement) pstmt);
                    if (oldServerPrepStmt != null && oldServerPrepStmt != pstmt) {
                        ((ServerPreparedStatement) oldServerPrepStmt).isCached = false;
                        ((ServerPreparedStatement) oldServerPrepStmt).setClosed(false);
                        ((ServerPreparedStatement) oldServerPrepStmt).realClose(true, true);
                    }
                }
            }
        }
    }

关于 PSCache

这个是关于 druid 的

争议较大的是 mysql 的时候是否开启 PSCache、下面 github 的 issue 是处于 open 状态

// com.alibaba.druid.pool.DruidPooledConnection#closePoolableStatement
public void closePoolableStatement(DruidPooledPreparedStatement stmt) throws SQLException {
  PreparedStatement rawStatement = stmt.getRawPreparedStatement();

  if (holder == null) {
    return;
  }

  if (stmt.isPooled()) {
    try {
      rawStatement.clearParameters();
    } catch (SQLException ex) {
      this.handleException(ex, null);
      if (rawStatement.getConnection().isClosed()) {
        return;
      }

      LOG.error("clear parameter error", ex);
    }
  }

  PreparedStatementHolder stmtHolder = stmt.getPreparedStatementHolder();
  stmtHolder.decrementInUseCount();
  // holder.isPoolPreparedStatements 对应上面配置的开关
  if (stmt.isPooled() && holder.isPoolPreparedStatements() && stmt.exceptionCount == 0) {
    // 放入缓存池子中
    holder.getStatementPool().put(stmtHolder);

    stmt.clearResultSet();
    holder.removeTrace(stmt);

    stmtHolder.setFetchRowPeak(stmt.getFetchRowPeak());
		
    stmt.setClosed(true); // soft set close
  } else if (stmt.isPooled() && holder.isPoolPreparedStatements()) {
    // the PreparedStatement threw an exception
    stmt.clearResultSet();
    holder.removeTrace(stmt);

    // 开启了PSCache但是这个stmt抛出过异常，直接从缓存中移除
    holder.getStatementPool()
      .remove(stmtHolder);
  } else {
    try {
      //Connection behind the statement may be in invalid state, which will throw a SQLException.
      //In this case, the exception is desired to be properly handled to remove the unusable connection from the pool.
      stmt.closeInternal();
    } catch (SQLException ex) {
      this.handleException(ex, null);
      throw ex;
    } finally {
      holder.getDataSource().incrementClosedPreparedStatementCount();
    }
  }
}

https://qsli.github.io/2020/05/05/cache-prep-stmts/

https://github.com/alibaba/druid/issues/2273

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