MySQL：添加索引不会提高性能

问：

所以我运行了以下内容：

EXPLAIN SELECT i.*
FROM (
    SELECT *,
           ROW_NUMBER() OVER (PARTITION BY symbol ORDER BY date DESC) AS row_num
    FROM indicators_5m
    WHERE ema_200 > 0 AND close > ema_200
) AS i
WHERE i.row_num <= 30;

我得到了以下输出：

以下是指数的详细信息：

以下是索引的详细信息：

show index from indicators_5m;

表结构

DESCRIBE indicators_5m;

id  int(11) NO  PRI NULL    auto_increment  
symbol  varchar(255)    YES MUL NULL        
open    float   YES     NULL        
high    float   YES     NULL        
low float   YES     NULL        
close   float   YES MUL NULL        
volume  int(11) YES     NULL        
date    datetime    YES     NULL        
rsi float   YES     NULL        
roc float   YES     NULL        
ema_7   float   YES     NULL        
ema_10  float   YES     NULL        
ema_50  float   YES     0       
ema_100 float   YES     0       
ema_200 float   YES     0       
ema_21  float   YES     NULL        
bb_upper    float   YES     NULL        
bb_middle   float   YES     NULL        
bb_lower    float   YES     NULL        
sma_20  float   YES     NULL        
sma_50  float   YES     NULL        
sma_100 float   YES     NULL        
pivot_points    float   YES     0       
resistance_1    float   YES     0       
support_1   float   YES     0       
resistance_2    float   YES     0       
support_2   float   YES     0       
resistance_3    float   YES     0       
support_3   float   YES     0       
macd    float   YES     0       
macd_histogram  float   YES     0       
macd_signal float   YES     0       


indicators_5m   0   PRIMARY 1   id  A   220146  NULL    NULL        BTREE           
indicators_5m   0   unique_symbol_date  1   symbol  A   258 NULL    NULL    YES BTREE           
indicators_5m   0   unique_symbol_date  2   date    A   220146  NULL    NULL    YES BTREE           
indicators_5m   1   idx_close_emas  1   close   A   55036   NULL    NULL    YES BTREE           
indicators_5m   1   idx_close_emas  2   ema_200 A   220146  NULL    NULL    YES BTREE           
indicators_5m   1   idx_close_emas  3   ema_21  A   220146  NULL    NULL    YES BTREE