JAVA MYSQL sql_calc_found_rows和found_rows()实践

一、背景

1.百万级数据库，数据量持续增加。每张数据表的字段数大于50（时间字段，分组字段，指标字段）

2.JDBCTemplate，java，mysql

二、问题描述

通过分析接口返回数据响应时间过长（通过某个分组字段搜索数据，响应时间长达30s）。

三、检查问题

检查代码，发现代码中运行了两句SQL语句，一句通过select查询数据，一句通过select count(1)来获取返回数据的总条数。

通过navicat查询语句对应的执行时间。

SELECT eventtime,smart_card_id,uevt_1000 FROM analytics_vhsession_user_event_info_day_201901 WHERE eventtime>='2019-01-01 00:00:00' AND eventtime<'2019-01-31 23:59:59' AND smart_card_id = '0382205801' ORDER BY eventtime asc LIMIT 0,1000 > OK > 时间: 10.603s SELECT count(1) FROM analytics_vhsession_user_event_info_day_201901 WHERE eventtime>='2019-01-01 00:00:00' AND eventtime<'2019-01-31 23:59:59' AND smart_card_id = '0382205801' > OK > 时间: 11.13s 

同样耗时10s+，所以想办法从select count(1)入手，减少SQL执行时间以达到减少响应时间的目的。

四、查询资料

通过查询资料，可以通过使用sql_calc_found_rows和found_rows()替代select count(1)。

通过navicat查询语句对应的执行时间。

SELECT sql_calc_found_rows eventtime,smart_card_id,uevt_1000 FROM analytics_vhsession_user_event_info_day_201901 WHERE eventtime>='2019-01-01 00:00:00' AND eventtime<'2019-01-31 23:59:59' AND smart_card_id = '0382205801' ORDER BY eventtime asc LIMIT 0,1000 > OK > 时间: 11.606s SELECT FOUND_ROWS() > OK > 时间: 0.004s 

相较之前的方案，响应时间可以减少10s以上，是一个值得尝试的方案。

五、优化尝试

根据之前的测试结果尝试进行代码优化，使用jdbcTemplate来调用两次query(),一次获取数据，一次获取对应的总条数。

//select sql_calc_found_rows String selectSQL = "select sql_calc_found_rows ..."; List<Map<String, Object>> data = jdbcTemplate.queryForList(selectSQL); //select found_rows() String selectTotalCountSQL = "select found_rows()"; Long totalCount = jdbcTemplate.queryForObject(selectTotalCountSQL, Long.class); 

但是在实际测试中遇到了jdbcTemplate.query("select found_rows()")返回的总条数与实际的总条数不一致的情况。 通过查询相应的资料，在一篇分享文档发现一点端倪，以下为资料原文：

we do this by opening a connection, running two SELECT queries, then closing the connection. This allows us to achieve the desired result that we need.

sql_calc_found_rows和found_rows()需要两句SQL在同一会话中，才能保证select found_rows()返回的总条数是上一句select sql_calc_found_rows对应的总条数

查看jdbcTemplate.query()底层代码实现。

public <T> T execute(StatementCallback<T> action) throws DataAccessException { Assert.notNull(action, "Callback object must not be null"); Connection con = DataSourceUtils.getConnection(getDataSource()); Statement stmt = null; try { Connection conToUse = con; if (this.nativeJdbcExtractor != null && this.nativeJdbcExtractor.isNativeConnectionNecessaryForNativeStatements()) { conToUse = this.nativeJdbcExtractor.getNativeConnection(con); } stmt = conToUse.createStatement(); applyStatementSettings(stmt); Statement stmtToUse = stmt; if (this.nativeJdbcExtractor != null) { stmtToUse = this.nativeJdbcExtractor.getNativeStatement(stmt); } T result = action.doInStatement(stmtToUse); handleWarnings(stmt); return result; } catch (SQLException ex) { // Release Connection early, to avoid potential connection pool deadlock // in the case when the exception translator hasn't been initialized yet. JdbcUtils.closeStatement(stmt); stmt = null; DataSourceUtils.releaseConnection(con, getDataSource()); con = null; throw getExceptionTranslator().translate("StatementCallback", getSql(action), ex); } finally { JdbcUtils.closeStatement(stmt); DataSourceUtils.releaseConnection(con, getDataSource()); } } 

jdbcTemplate每次执行query()都会从连接池中获取连接

Connection con = DataSourceUtils.getConnection(getDataSource()) 

执行完成后释放连接

DataSourceUtils.releaseConnection(con, getDataSource()); 

不能保证两次query()在一个会话中(同一个Connection)。

六、优化实践

优化方案：不使用JDBCTemplate中的query()方法，自己实现具体逻辑。通过DataSourceUtils.getConnection(jdbcTemplate.getDataSource())获取会话，使用Statement来执行两次SQL后，再通过DataSourceUtils.releaseConnection(conn, jdbcTemplate.getDataSource());释放会话，保证两句SQL在同一会话中。

public PagedArrayList getDataAndTotalCount(String sql){ Connection conn = null; Statement statement = null; ResultSet rs = null; ResultSet rs1 = null; long totalCount = 0L; PagedArrayList data = new PagedArrayList(); try { conn = DataSourceUtils.getConnection(jdbcTemplate.getDataSource()); conn.setAutoCommit(true); statement = conn.createStatement(); rs = statement.executeQuery(sql); ResultSetMetaData md = rs.getMetaData(); //获得结果集结构信息,元数据 int columnCount = md.getColumnCount(); //获得列数 while (rs.next()) { Map<String,Object> rowData = new HashMap<String,Object>(); for (int i = 1; i <= columnCount; i++) { rowData.put(md.getColumnName(i), rs.getObject(i)); } data.add(rowData); } String totalCountSQL = "select found_rows() AS total_count"; rs1 = statement.executeQuery(totalCountSQL); while (rs1.next()){ totalCount = rs1.getLong("total_count"); } data.totalCount = totalCount; } catch (Exception e) { slf4jLogger.error("getDataAndTotalCount() error:", e); } finally { //关闭资源 JdbcUtils.closeResultSet(rs); JdbcUtils.closeResultSet(rs1); JdbcUtils.closeStatement(statement); //释放资源 DataSourceUtils.releaseConnection(conn, jdbcTemplate.getDataSource()); } return data; } 

七、参考文档

https://www.contradodigital.com/2018/01/06/how-to-use-sql_calc_found_rows-and-found_rows-with-limit-and-offset-in-a-mysql-query-using-java-and-jdbc/