查询中的Oracle和SQLServer函数评估

Question

假设我在Oracle中的select或where子句上有一个函数调用,如下所示:

select a, b, c, dbms_crypto.hash(utl_raw.cast_to_raw('HELLO'),3)
  from my_table

可以为MS SQLServer构建类似的示例.

每种情况下的预期行为是什么？

是HASH函数去为表中的每一行调用一次,或将DBMS足够聪明来调用函数只有一次,因为它是恒定的参数和无功能的副作用？

非常感谢.

Answer 1

Oracle的答案取决于它.将为每个选定的行调用该函数,除非函数标记为"确定性",在这种情况下,它只会被调用一次.

CREATE OR REPLACE PACKAGE TestCallCount AS
    FUNCTION StringLen(SrcStr VARCHAR) RETURN INTEGER;
    FUNCTION StringLen2(SrcStr VARCHAR) RETURN INTEGER DETERMINISTIC;
    FUNCTION GetCallCount RETURN INTEGER;
    FUNCTION GetCallCount2 RETURN INTEGER;
END TestCallCount;

CREATE OR REPLACE PACKAGE BODY TestCallCount AS
    TotalFunctionCalls INTEGER := 0;
    TotalFunctionCalls2 INTEGER := 0;

    FUNCTION StringLen(SrcStr VARCHAR) RETURN INTEGER AS
    BEGIN
        TotalFunctionCalls := TotalFunctionCalls + 1;
        RETURN Length(SrcStr);
    END;
    FUNCTION GetCallCount RETURN INTEGER AS
    BEGIN
        RETURN TotalFunctionCalls;
    END;

    FUNCTION StringLen2(SrcStr VARCHAR) RETURN INTEGER DETERMINISTIC AS
    BEGIN
        TotalFunctionCalls2 := TotalFunctionCalls2 + 1;
        RETURN Length(SrcStr);
    END;
    FUNCTION GetCallCount2 RETURN INTEGER AS
    BEGIN
        RETURN TotalFunctionCalls2;
    END;

END TestCallCount;




SELECT a,TestCallCount.StringLen('foo') FROM(
    SELECT 0 as a FROM dual
    UNION
    SELECT 1 as a FROM dual
    UNION
    SELECT 2 as a FROM dual
);

SELECT TestCallCount.GetCallCount() AS TotalFunctionCalls FROM dual;

输出:

A                      TESTCALLCOUNT.STRINGLEN('FOO') 
---------------------- ------------------------------ 
0                      3                              
1                      3                              
2                      3                              

3 rows selected


TOTALFUNCTIONCALLS     
---------------------- 
3                      

1 rows selected

因此,在第一种情况下,StringLen()函数被调用了三次.现在用StringLen2()执行时表示确定性:

SELECT a,TestCallCount.StringLen2('foo') from(
    select 0 as a from dual
    union
    select 1 as a from dual
    union
    select 2 as a from dual
);

SELECT TestCallCount.GetCallCount2() AS TotalFunctionCalls FROM dual;

结果:

A                      TESTCALLCOUNT.STRINGLEN2('FOO') 
---------------------- ------------------------------- 
0                      3                               
1                      3                               
2                      3                               

3 rows selected

TOTALFUNCTIONCALLS     
---------------------- 
1                      

1 rows selected

所以StringLen2()函数只被调用一次,因为它被标记为确定性.

对于未标记为确定性的函数,您可以通过修改查询来解决此问题:

select a, b, c, hashed
  from my_table
cross join (
  select dbms_crypto.hash(utl_raw.cast_to_raw('HELLO'),3) as hashed from dual
);