postgresql - PL/pgSQL block 是如何编译的？

Question

   do $$
  declare 
   tm1 timestamp without time zone;
   tm2 timestamp without time zone;
  begin
     select localtimestamp(0) into tm1;
     for i in 1..200000000 loop
        --just waiting several second
     end loop;
     select localtimestamp(0) into tm2;

      raise notice '% ; %', tm1, tm2;
  end;
  $$ language plpgsql

为什么给这个过程 tm1 和 tm2 相同的值？

这段代码不是一步步执行的吗？

Answer 1

From the manual

These SQL-standard functions all return values based on the start time of the current transaction [...] Since these functions return the start time of the current transaction, their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp

^{(强调我的)}

您可能需要 clock_timestamp()