Postgresql different queries Why the same explain?

Question

Query 1:

    constructor(props) {
         super(props);
         this.state = { controlledDate: '' };
         this.handleChangeDate = this.handleChangeDate.bind(this);
    }

    ...

    render() {
       ...

       <TableRow>
           TableRowColumn>{this.state.controlledDate || New Date(Date.now())}</TableRowColumn>
       </TableRow>
    }

Query 2:

select c_bh,c_xzdm,c_twhbm,d_tjrq 
from (
   select c_bh,c_xzdm,c_twhbm,d_tjrq 
   from t_table 
   order by d_tjrq desc
) t1  
limit 10

The same plan:

select c_bh,c_xzdm,c_twhbm,d_tjrq 
from t_table 
order by d_tjrq desc 
limit 10

Answer 1

在第一种情况下，PostgreSQL“扁平化”子查询，以便更好地进行优化。结果等同于第二个查询。

如果您想避免这种情况，请将OFFSET 0之类的优化屏障放入子查询中。