sql like走索引吗

标签可用于通过 LIKE 前缀匹配来提高性能。
全表搜索；标签无效；需要进行全表扫描。
数据库之间的索引实现存在差异。
详细信息请参阅文档或测试。

给自己计时。

sql中怎么查看索引状态 查看索引状态的几种实用方法

For index status, refer to this database manual.
SQLServer:
System view index query: sys.indexes+sys.index_columns
Look at the selectivity of statistical information: DBCC SHOW_STATISTICS('table name', 'index name')
MySQL:
SHOW INDEX FROM '表名' 查看索引
EXPLAIN to see whether to use the index
PostgreSQL:
pg_indexes查看索引
auto_explain extension to display execution plan
Fragmentation processing: SQL Server: Use REBUILD heavily, REORGANIZE lightly MySQL: OPTIMIZE TABLE PostgreSQL: REINDEX
Monitoring: SQL Server: Profiler+Execution Plan MySQL：慢查询日志（slow_query_log=ON） PostgreSQL: pg_stat_user_indexes
Optimization strategy:
Creation times: WHERE/JOIN/GROUP BY column, highly selective column
Avoid abuse: slow down index writing, clean up redundant indexes
Type selection: B-tree (default), Hash (equivalent query), full text (text)
FAQ:
The index has no effect: the function rewrites the column, low selectivity and the optimizer does not select the large amount of data.
Too many indexes: wasted space, slow writing, slow index selection by the optimizer
Check for yourself.

SQL如何实现全文搜索_SQL全文搜索的实现方法

SQL中EXISTS子查询的优化技巧 EXISTS子查询提升性能的编写方式

使用索引优化：在order.customer_id上创建索引，例如：CREATE INDEX idx_orders_customer_id ONorders(customer_id)。

简化子查询逻辑：使用 GROUP BY 和 HAVING 代替多级 EXISTS，例如： SELECT c.customer_id, c.customer_name FROM customer c JOIN Orders o ON c.customer_id = o.customer_id GROUP BY c.customer_id, c.customer_name HAVING COUNT(DISTINCT o.order_id) >= 2
使用 JOIN而不是 EXISTS：使用 JOIN 代替 EXISTS，例如： SELECT DISTINCT c.customer_id, c.customer_name FROMcustomers c COMBINE order o ON c.customer_id = o.customer_id。

首先使用SELECT1 ：将SELECT替换为SELECT1 ，例如：SELECT customer_id, customer_name FROM customer WHERE EXISTS (SELECT1 FROMorders WHEREorders.customer_id = customer.customer_id)。

执行计划分析。
使用 EXPLAIN 解析查询，例如：EXPLAIN SELECT customer_id, customer_name FROMcustomers WHERE EXISTS (SELECT1 FROMorders WHEREorders.customer_id = customer_id)。

避免全表扫描。