文章目录
- where 1=1
- 问题描述
- 错误实现
- 正确实现
- 性能对比测试
where 1=1
问题描述
在动态 SQL 拼接场景中，开发者常使用 WHERE 1=1 简化条件拼接逻辑（避免处理首个条件的 AND）。理论上，数据库优化器会忽略 1=1，但字符串拼接可能带来性能损耗，部分数据库优化器可能无法完全优化 1=1，导致索引失效或全表扫描。建议使用<where> 标签替代 WHERE 1=1 。
错误实现
select *
from job_info
where 1=1
<if test="jobName != null and jobName != '' "><bind name="jobNameBind" value="'%' + jobName + '%'"/>and job_name like #{jobNameBind}
</if>
正确实现
select *
from job_info
<where>
<if test="jobName != null and jobName != '' "><bind name="jobNameBind" value="'%' + jobName + '%'"/>job_name like #{jobNameBind}
</if>
</where>
性能对比测试
-
数据库：mysql-8.0.29
-
EXPLAIN测试对比：
EXPLAIN SELECT * FROM koca_order WHERE 1=1 AND order_id > 1012EXPLAIN SELECT * FROM koca_order WHERE order_id > 1012
| id | select_type | type | possible_keys | rows | filtered | Extra |
|---|---|---|---|---|---|---|
| 1 | SIMPLE | ALL | PRIMARY | 600 | 33.33 | Using where |
- JMH测试：
@BenchmarkMode(Mode.Throughput)
@OutputTimeUnit(TimeUnit.MILLISECONDS)
@State(Scope.Benchmark)
@Fork(3)
@Warmup(iterations = 5, time = 2)
@Measurement(iterations = 5, time = 5)
@Threads(16)
public class WhereConditionBenchmark {private List<String> conditions;@Setuppublic void setup() {// 模拟 5 个动态查询条件conditions = Arrays.asList("age > 18", "status = 1", "name LIKE 'John%'", "city = 'NY'", "score >= 60");}/*** 原始实现：使用 WHERE 1=1 简化拼接*/@Benchmarkpublic void withDummyCondition(Blackhole bh) {StringBuilder sql = new StringBuilder("SELECT * FROM users WHERE 1=1");for (String cond : conditions) {sql.append(" AND ").append(cond);}bh.consume(sql.toString()); // 避免 JIT 优化}/*** 优化实现：直接拼接 WHERE 条件*/@Benchmarkpublic void withoutDummyCondition(Blackhole bh) {StringBuilder sql = new StringBuilder("SELECT * FROM users");boolean first = true;for (String cond : conditions) {if (first) {sql.append(" WHERE ");first = false;} else {sql.append(" AND ");}sql.append(cond);}bh.consume(sql.toString());}}
测试结果：
Benchmark Mode Cnt Score Error Units
TT.WhereConditionBenchmark.withDummyCondition thrpt 15 16509.986 ± 3028.386 ops/ms
TT.WhereConditionBenchmark.withoutDummyCondition thrpt 15 20124.583 ± 420.773 ops/ms
测试结论：
移除冗余的 WHERE 1=1 后，吞吐量提升约 21.9%，说明 1=1 对动态 SQL 生成性能有明显影响。