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Lobster25
Asked:2024-08-06 16:27:45 +0000 UTC2024-08-06 16:27:45 +0000 UTC

PostgreSQL 表达式计算多少次？

  • 772

t.col + 1您能否告诉我此查询中的表达式将被物理计数多少次？

select t.col + 1 as first, 
       t.col + 1 as second,
       case
           when t.col + 1 = 10
               then X
           when t.col + 1 = 20
               then Y
       end as caseVal
from table
sql

2 个回答

  1. Best Answer
    psql (18devel)
Type "help" for help.

postgres=# select pg_backend_pid();
 pg_backend_pid 
----------------
           8799
(1 row)

postgres=# with t as (select 1 as col, 5 as x, 6 as y) 
   select t.col + 1 as first,
       t.col + 1 as second,
       case                   
           when t.col + 1 = 10
               then X
           when t.col + 1 = 20
               then Y
       end as caseVal
from t;
 first | second | caseval 
-------+--------+---------
     2 |      2 |        
(1 row)

gdb -p 8799

(gdb) break int4pl
Breakpoint 1 at 0x556c1f530651: file ../../../../src/include/postgres.h, line 204.
(gdb) c
Continuing.

Breakpoint 1, int4pl (fcinfo=0x556c1ffacba0) at int.c:794
794     int32       result;
(gdb) c
Continuing.

Breakpoint 1, int4pl (fcinfo=0x556c1ffacba0) at int.c:794
794     int32       result;
(gdb) c
Continuing.

Breakpoint 1, int4pl (fcinfo=0x556c1ffacba0) at int.c:794
794     int32       result;
(gdb) c
Continuing.

Breakpoint 1, int4pl (fcinfo=0x556c1ffacba0) at int.c:794
794     int32       result;
(gdb) c
Continuing.

    col != 9 则四次，col = 9 则三次。

    • 3

  2. 该表达式将被执行三次。

    1. 对该表达式t.col + 1求值一次以计算该列的值first。
    2. 再次计算表达式t.col + 1以计算列的值second。
    3. 该表达式t.col + 1在条件语句中被第三次求值case。
    • 1

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