MyBatis批量插入幾千條數據慎用foreach
近日，項目中有一個耗時較長的 Job 存在 CPU 占用過高的問題。
<insert id="batchInsert" parameterType="java.util.List">
insert into USER (id, name) values
<foreach collection="list" item="model" index="index" separator=",">
(#{model.id}, #{model.name})
</foreach>
</insert>
這個方法提升批量插入速度的原理是，將傳統的：
INSERT INTO `table1` (`field1`, `field2`) VALUES ("data1", "data2");
INSERT INTO `table1` (`field1`, `field2`) VALUES ("data1", "data2");
INSERT INTO `table1` (`field1`, `field2`) VALUES ("data1", "data2");
INSERT INTO `table1` (`field1`, `field2`) VALUES ("data1", "data2");
INSERT INTO `table1` (`field1`, `field2`) VALUES ("data1", "data2");
轉化為：
INSERT INTO `table1` (`field1`, `field2`) VALUES ("data1", "data2"),
("data1", "data2"),
("data1", "data2"),
("data1", "data2"),
("data1", "data2");
在 MySql Docs：https://dev.mysql.com/doc/refman/5.6/en/insert-optimization.html中也提到過這個 trick，如果要優化插入速度時，可以將許多小型操作組合到一個大型操作中。理想情況下，這樣可以在單個連接中一次性發送許多新行的數據，并將所有索引更新和一致性檢查延遲到最后才進行。
乍看上去這個 foreach 沒有問題，但是經過項目實踐發現，當表的列數較多（20+），以及一次性插入的行數較多（5000+）時，整個插入的耗時十分漫長，達到了 14 分鐘，這是不能忍的。在[資料]https://stackoverflow.com/questions/19682414/how-can-mysql-insert-millions-records-fast中也提到了一句話：
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Of course don't combine ALL of them, if the amount is HUGE. Say you have 1000 rows you need to insert, then don't do it one at a time. You shouldn't equally try to have all 1000 rows in a single query. Instead break it into smaller sizes.
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它強調，當插入數量很多時，不能一次性全放在一條語句里。可是為什么不能放在同一條語句里呢？這條語句為什么會耗時這么久呢？我查閱了[資料]https://stackoverflow.com/questions/32649759/using-foreach-to-do-batch-insert-with-mybatis/40608353發現：
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「Insert inside Mybatis foreach is not batch」, this is a single (could become giant) SQL statement and that brings drawbacks:
- some database such as Oracle here does not support.
- in relevant cases: there will be a large number of records to insert and the database configured limit (by default around 2000 parameters per statement) will be hit, and eventually possibly DB stack error if the statement itself become too large.
Iteration over the collection must not be done in the mybatis XML. Just execute a simple Insertstatement in a Java Foreach loop. 「The most important thing is the session Executor type」.
SqlSession session = sessionFactory.openSession(ExecutorType.BATCH);
for (Model model : list) {
session.insert("insertStatement", model);
}
session.flushStatements();
Unlike default ExecutorType.SIMPLE, the statement will be prepared once and executed for each record to insert.
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從[資料]https://blog.csdn.net/wlwlwlwl015/article/details/50246717中可知，默認執行器類型為Simple，會為每個語句創建一個新的預處理語句，也就是創建一個「PreparedStatement」對象。在我們的項目中，會不停地使用批量插入這個方法，而因為MyBatis對于含有<foreach>的語句，無法采用緩存，那么在每次調用方法時，都會重新解析sql語句。
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Internally, it still generates the same single insert statement with many placeholders as the JDBC code above.
MyBatis has an ability to cache PreparedStatement, but this statement cannot be cached because it contains <foreach /> element and the statement varies depending on the parameters.
As a result, MyBatis has to 1) evaluate the foreach part and 2) parse the statement string to build parameter mapping [1] on every execution of this statement.
And these steps are relatively costly process when the statement string is big and contains many placeholders.
[1] simply put, it is a mapping between placeholders and the parameters.
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從上述[資料] http://blog.harawata.net/2016/04/bulk-insert-multi-row-vs-batch-using.html 可知，耗時就耗在，由于我 foreach 后有 5000+ 個 values，所以這個PreparedStatement 特別長，包含了很多占位符，對于占位符和參數的映射尤其耗時。并且，查閱相關[資料]https://www.red-gate.com/simple-talk/sql/performance/comparing-multiple-rows-insert-vs-single-row-insert-with-three-data-load-methods 可知，values 的增長與所需的解析時間，是呈指數型增長的。
所以，如果非要使用 foreach 的方式來進行批量插入的話，可以考慮減少一條 insert 語句中 values 的個數，最好能達到上面曲線的最底部的值，使速度最快。一般按[經驗]https://stackoverflow.com/questions/7004390/java-batch-insert-into-mysql-very-slow來說，一次性插 20~50 行數量是比較合適的，時間消耗也能接受。
重點來了。上面講的是，如果非要用<foreach>的方式來插入，可以提升性能的方式。而實際上，MyBatis文檔中寫批量插入的時候，是推薦使用另外一種方法。（可以看http://www.mybatis.org/mybatis-dynamic-sql/docs/insert.html中 「Batch Insert Support」 標題里的內容）
SqlSession session = sqlSessionFactory.openSession(ExecutorType.BATCH);
try {
SimpleTableMapper mapper = session.getMapper(SimpleTableMapper.class);
List<SimpleTableRecord> records = getRecordsToInsert(); // not shown
BatchInsert<SimpleTableRecord> batchInsert = insert(records)
.into(simpleTable)
.map(id).toProperty("id")
.map(firstName).toProperty("firstName")
.map(lastName).toProperty("lastName")
.map(birthDate).toProperty("birthDate")
.map(employed).toProperty("employed")
.map(occupation).toProperty("occupation")
.build()
.render(RenderingStrategy.MYBATIS3);
batchInsert.insertStatements().stream().forEach(mapper::insert);
session.commit();
} finally {
session.close();
}
即基本思想是將MyBatis session的executor type設為 「Batch」，然后多次執行插入語句。就類似于 JDBC 的下面語句一樣。
Connection connection = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/mydb?useUnicode=true&characterEncoding=UTF-8&useServerPrepStmts=false&rewriteBatchedStatements=true","root","root");
connection.setAutoCommit(false);
PreparedStatement ps = connection.prepareStatement(
"insert into tb_user (name) values(?)");
for (int i = 0; i < stuNum; i++) {
ps.setString(1,name);
ps.addBatch();
}
ps.executeBatch();
connection.commit();
connection.close();
經過試驗，使用了ExecutorType.BATCH的插入方式，性能顯著提升，不到 2s 便能全部插入完成。
總結一下，如果 MyBatis 需要進行批量插入，推薦使用ExecutorType.BATCH的插入方式，如果非要使用<foreach>的插入的話，需要將每次插入的記錄控制在 20~50 左右。
