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> 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 execute(StatementCallback 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 rowData = new HashMap();
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/