数据:单表数据25万条。
select * from cf_qb_info limit 0,20
select * from cf_qb_info limit 200000,20
提示:如果这一步很慢,count(1) 查询总数应该也会很慢-解决方式:请为主键加上unique索引。
-- 主键ID字段:NUMID select NUMID from cf_qb_info limit 200000,20
基于第三步的缺陷(只能查出ID信息),我们可以先查出分页数据的ID,在根据ID查询数据。
select NUMID from cf_qb_info LIMIT 200000,20
select * from cf_qb_info where NUMID in ( '330681650000202108180227345510', '330681650000202108171031534500', '330681650000202108190251532141', '330681650000202108200246376830', '330681650000202108210229398665', '330681650000202108220236113895', '330681650000202108230230034133', '330681650000202108231017279739', '330681650000202108231043456276', '330681650000202108231051404340', '330681650000202108240237397251', '330681650000202108250221489228', '330681650000202108250241536726', '330681650000202108260253039326', '330681650000202108270216016138', '330681650000202108280234013754', '330681650000202108290230029720', '330681650000202108300255579204', '330681650000202108310234184991', '330681650000202109010237315937' );
两步合成一步SQL耗时:11.9秒;这一步着实出乎了我的意料。
select * from cf_qb_info where NUMID in ( select NUMID from (select NUMID from cf_qb_info LIMIT 200000,20) as t );
鉴于这个结果:我们可以在程序里分成两步进行分页查询。
这一步是对第四步的优化,毕竟两条SQL还需要码代码。利用join 两条SQL合成一条。
SELECT * FROM cf_qb_info a JOIN ( SELECT NUMID FROM cf_qb_info LIMIT 200000, 20 ) b ON a.NUMID = b.NUMID
原理是使用了BeanSearcher的sql拦截器对SQL进行拦截改造。
①改造Bean
②注入Sql拦截器
package com.ciih.qbbs.config; import cn.hutool.core.util.ReUtil; import cn.hutool.core.util.StrUtil; import com.baomidou.mybatisplus.annotation.TableId; import com.ejlchina.searcher.SearchSql; import com.ejlchina.searcher.SqlInterceptor; import com.ejlchina.searcher.SqlSnippet; import com.ejlchina.searcher.param.FetchType; import org.springframework.stereotype.Component; import java.lang.reflect.Field; import java.util.List; import java.util.Map; /** * BeanSearcher的Sql拦截器:优化深度分页 * * @author sunziwen */ @Component public class SqlInterceptorImpl implements SqlInterceptor { @Override public <T> SearchSql<T> intercept(SearchSql<T> searchSql, Map<String, Object> paraMap, FetchType fetchType) { /** * 改造思路 * * <> * 前:SELECT * FROM table1 t1 LIMIT 200000,20; * 后:SELECT * FROM table1 t1 JOIN ( SELECT id FROM table1 LIMIT 200000, 20 ) t99 ON t1.id = t99.id; * </> */ Field[] fields = searchSql.getBeanMeta().getBeanClass().getDeclaredFields(); String primaryColumnName = null; for (Field field : fields) { //这里使用了mybatis_plus的注解作为主键标识 TableId tableId = field.getAnnotation(TableId.class); if (tableId != null) { if (!"".equals(tableId.value())) { primaryColumnName = tableId.value(); } else { //驼峰转下划线 primaryColumnName = StrUtil.toUnderlineCase(field.getName()); } } } //如果没有主键标识,则不能进行SQL优化。 if (primaryColumnName == null) { return searchSql; } //正则表达式获取where之后语句 List<String> limits = ReUtil.findAll("where[\\s\\S]*limit[ ]+[?]{1}[ ]*,[ ]+[?]{1}", searchSql.getListSqlString(), 0); //如果不分页,则不进行SQL优化,即语句中没有limit关键字不优化。 if (limits.size() == 0) { return searchSql; } //表名小片段 SqlSnippet tableSnippet = searchSql.getBeanMeta().getTableSnippet(); //合成子查询SQL String inSql = "JOIN ( SELECT " + primaryColumnName + " FROM " + tableSnippet.getSql() + " " + limits.get(0) + " ) t99 ON t1." + primaryColumnName + " = t99." + primaryColumnName + ";"; //合成整条SQL String replace = searchSql.getListSqlString().replace(limits.get(0), inSql); //替换 searchSql.setListSqlString(replace); return searchSql; } }
以上为个人经验,希望能给大家一个参考,也希望大家多多支持脚本之家。
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