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近日，項(xiàng)目中有一個(gè)耗時(shí)較長的Job存在CPU占用過高的問題巨税，經(jīng)排查發(fā)現(xiàn)蟋定，主要時(shí)間消耗在往MyBatis中批量插入數(shù)據(jù)。mapper configuration是用foreach循環(huán)做的草添，差不多是這樣驶兜。（由于項(xiàng)目保密，以下代碼均為自己手寫的demo代碼）

<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(gè)方法提升批量插入速度的原理是远寸，將傳統(tǒng)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(zhuǎn)化為：

INSERT INTO `table1` (`field1`, `field2`) 
VALUES ("data1", "data2"),
("data1", "data2"),
("data1", "data2"),
("data1", "data2"),
("data1", "data2");

在MySql Docs中也提到過這個(gè)trick抄淑，如果要優(yōu)化插入速度時(shí)，可以將許多小型操作組合到一個(gè)大型操作中驰后。理想情況下肆资，這樣可以在單個(gè)連接中一次性發(fā)送許多新行的數(shù)據(jù)，并將所有索引更新和一致性檢查延遲到最后才進(jìn)行灶芝。

乍看上去這個(gè)foreach沒有問題迅耘，但是經(jīng)過項(xiàng)目實(shí)踐發(fā)現(xiàn)贱枣，當(dāng)表的列數(shù)較多（20+），以及一次性插入的行數(shù)較多（5000+）時(shí)颤专，整個(gè)插入的耗時(shí)十分漫長纽哥，達(dá)到了14分鐘，這是不能忍的栖秕。在資料中也提到了一句話：

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.

它強(qiáng)調(diào)春塌，當(dāng)插入數(shù)量很多時(shí)，不能一次性全放在一條語句里簇捍≈豢牵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(shí)這么久呢暑塑？我查閱了資料發(fā)現(xiàn)：

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);

Unlike default ExecutorType.SIMPLE, the statement will be prepared once and executed for each record to insert.

從資料中可知吼句，默認(rèn)執(zhí)行器類型為Simple，會為每個(gè)語句創(chuàng)建一個(gè)新的預(yù)處理語句事格，也就是創(chuàng)建一個(gè)PreparedStatement對象惕艳。在我們的項(xiàng)目中，會不停地使用批量插入這個(gè)方法驹愚，而因?yàn)镸yBatis對于含有<foreach>的語句远搪，無法采用緩存，那么在每次調(diào)用方法時(shí)逢捺，都會重新解析sql語句谁鳍。

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.

從上述資料可知，耗時(shí)就耗在劫瞳，由于我foreach后有5000+個(gè)values倘潜，所以這個(gè)PreparedStatement特別長，包含了很多占位符志于，對于占位符和參數(shù)的映射尤其耗時(shí)窍荧。并且，查閱相關(guān)資料可知恨憎，values的增長與所需的解析時(shí)間蕊退，是呈指數(shù)型增長的。

所以憔恳，如果非要使用 foreach 的方式來進(jìn)行批量插入的話瓤荔，可以考慮減少一條 insert 語句中 values 的個(gè)數(shù)，最好能達(dá)到上面曲線的最底部的值钥组，使速度最快输硝。一般按經(jīng)驗(yàn)來說，一次性插20~50行數(shù)量是比較合適的程梦，時(shí)間消耗也能接受点把。

重點(diǎn)來了橘荠。上面講的是，如果非要用<foreach>的方式來插入郎逃，可以提升性能的方式哥童。而實(shí)際上，MyBatis文檔中寫批量插入的時(shí)候褒翰，是推薦使用另外一種方法贮懈。（可以看 http://www.mybatis.org/mybatis-dynamic-sql/docs/insert.html 中 Batch Insert Support 標(biāo)題里的內(nèi)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(shè)為 Batch 喇闸，然后多次執(zhí)行插入語句否灾。就類似于JDBC的下面語句一樣软能。

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(jīng)過試驗(yàn)染坯，使用了 ExecutorType.BATCH 的插入方式，性能顯著提升翔忽，不到 2s 便能全部插入完成呼渣。

總結(jié)一下蔑祟，如果MyBatis需要進(jìn)行批量插入早敬，推薦使用 ExecutorType.BATCH 的插入方式忌傻，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 左右搁嗓。