很少有开发者意识到一个问题，那就是在SQL中使用IN列表时可能会遇到 "游标缓存争夺 "或 "执行计划缓存争夺 "的问题。在以前的文章中详细描述了这个问题，可以概括为这样。
所有这些都是不同的SQL查询，需要在有强大计划缓存的RDBMS（例如Db2、Oracle、SQL Server）中被解析/计划/缓存为可能不同的执行计划：
SELECT * FROM t WHERE id IN (?);
SELECT * FROM t WHERE id IN (?, ?);
SELECT * FROM t WHERE id IN (?, ?, ?);
SELECT * FROM t WHERE id IN (?, ?, ?, ?);
SELECT * FROM t WHERE id IN (?, ?, ?, ?, ?);
虽然这在开发者机器上不是 问题，但在生产中会产生重大 问题。我见过这种情况，在高峰负载期间，整个Oracle实例都会瘫痪。虽然RDBMS供应商应该努力避免这可能导致的严重问题，但你可以使用我们在jOOQ发明的一个技巧来解决它（Hibernate现在也有）。
IN 列表填充
这个技巧非常简单。只要将你的IN 列表 "填充 "到最接近的2 的幂数，然后重复最后一个值，直到结束：
SELECT * FROM t WHERE id IN (?); -- Left as it is
SELECT * FROM t WHERE id IN (?, ?); -- Left as it is
SELECT * FROM t WHERE id IN (?, ?, ?, ?); -- Padded 3 to 4
SELECT * FROM t WHERE id IN (?, ?, ?, ?); -- Left as it is
SELECT * FROM t WHERE id IN (?, ?, ?, ?, ?, ?, ?, ?); -- Padded 5 to 8
这真的是一个黑客，有更好的解决方案来避免这个问题，包括使用数组或临时表，但你的生产系统可能已经瘫痪了，你需要一个快速修复。
从jOOQ 3.9（2016年底）开始，jOOQ已经支持IN 列表填充多年了，但随着相对较新的解析器和ParsingConnection ，你现在也可以在你的非jOOQ系统中对任何任意的SQL查询应用这一技术。这里有一个简单的例子：
// Any arbitrary JDBC Connection is wrapped by jOOQ here and replaced
// by a "ParsingConnection", which is also a JDBC Connection
DSLContext ctx = DSL.using(connection);
ctx.settings().setInListPadding(true);
Connection c = ctx.parsingConnection();
// Your remaining code is left untouched. It is unaware of jOOQ
for (int i = 0; i < 10; i++) {
try (PreparedStatement s = c.prepareStatement(
// This alone is reason enough to use jOOQ instead,
// but one step at a time :)
"select 1 from dual where 1 in (" +
IntStream.rangeClosed(0, i)
.mapToObj(x -> "?")
.collect(Collectors.joining(", ")) +
")")
) {
for (int j = 0; j <= i; j++)
s.setInt(j + 1, j + 1);
try (ResultSet rs = s.executeQuery()) {
while (rs.next())
System.out.println(rs.getInt(1));
}
}
}
上面的例子只是生成和运行了10个这种形式的查询：
select 1 from dual where 1 in (?)
select 1 from dual where 1 in (?, ?)
select 1 from dual where 1 in (?, ?, ?)
select 1 from dual where 1 in (?, ?, ?, ?)
select 1 from dual where 1 in (?, ?, ?, ?, ?)
select 1 from dual where 1 in (?, ?, ?, ?, ?, ?)
select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?)
select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?)
select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?, ?)
select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
但这并不是正在执行的内容。在DEBUG日志中，我们可以看到以下内容：
Translating from : select 1 from dual where 1 in (?)
Translating to : select 1 from dual where 1 in (?)
Translating from : select 1 from dual where 1 in (?, ?)
Translating to : select 1 from dual where 1 in (?, ?)
Translating from : select 1 from dual where 1 in (?, ?, ?)
Translating to : select 1 from dual where 1 in (?, ?, ?, ?)
Translating from : select 1 from dual where 1 in (?, ?, ?, ?)
Translating to : select 1 from dual where 1 in (?, ?, ?, ?)
Translating from : select 1 from dual where 1 in (?, ?, ?, ?, ?)
Translating to : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?)
Translating from : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?)
Translating to : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?)
Translating from : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?)
Translating to : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?)
Translating from : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?)
Translating to : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?)
Translating from : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?, ?)
Translating to : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
Translating from : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
Translating to : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
就这样，我们的传统应用程序又可以在生产中运行了，你将有时间来更彻底地解决这个问题。
结论
虽然jOOQ主要是一个内部DSL，用于在Java中编写类型安全的嵌入式SQL，但你也可以在任何基于JDBC的应用程序中使用它做很多其他事情。上面的例子是使用ParsingConnection ，它可以解析你所有的SQL语句，并将它们翻译/转换为任何其他东西，包括其他方言。