R sqldf-循环内/外索引

我使用sqldf函数反复将表的子集与其自身连接。重复过程在for循环中发生。我已经读到,添加索引可以提高这些连接的性能 here .

我的问题是——如果我在循环中重复这样做,这是否意味着每次循环执行时都必须重新创建索引,或者有没有办法让索引在循环外“持久化”,但在循环内使用?

换句话说,我只看过这个版本:

for(i in 1:10){
   df1 <- sqldf(c('create index...','select * from table1'))
}

有没有这样的方法:

df1 <- sqldf('create index...') # create index outside of loop

for(i in 1:10){
   df2 <- sqldf('select * from t1 left join t2 on t1.col1 = t2.col1')
}

编辑:

> sqldf() 
NULL
> 
> sqldf("create index idx on iris(Species)") ## 
data frame with 0 columns and 0 rows
> sqldf("select count(*) from main.iris where Species = 'virginica'") ##
Error in rsqlite_send_query(conn@ptr, statement) : 
  no such table: main.iris
> sqldf("select count(*) from main.iris where Species <> 'virginica'") ##
Error in rsqlite_send_query(conn@ptr, statement) : 
  no such table: main.iris
> 
> sqldf()
<SQLiteConnection>
  Path: :memory:
  Extensions: TRUE
> 

编辑2:

> sqldf() 
NULL
> # close an old connection if it exists
>    if (!is.null(getOption("sqldf.connection"))) sqldf()
> sqldf() 
<SQLiteConnection>
  Path: :memory:
  Extensions: TRUE
> sqldf("create index idx on iris(Species)") ## 
data frame with 0 columns and 0 rows
> sqldf("select count(*) from main.iris where Species = 'virginica'") ##
  count(*)
1       50
> sqldf("select count(*) from main.iris where Species <> 'virginica'") ##
  count(*)
1      100
> sqldf() 
NULL