some_proc(sysdate, sysdate); - sysdate vil IKKE altid være det samme, når det bruges i en PL/SQL-sætning
select some_func(sysdate, sysdate) from dual; - sysdate vil altid være den samme, når den bruges i en SQL-sætning (selvom den SQL-sætning kalder PL/SQL)
Fra Læsekonsistens på erklæringsniveau afsnittet i Concepts-guiden:"Oracle håndhæver altid læsekonsistens på erklæringsniveau. Dette garanterer, at alle de data, der returneres af en enkelt forespørgsel, kommer fra et enkelt tidspunkt – det tidspunkt, hvor forespørgslen begyndte."
Den side antyder på en måde, at det samme ikke er tilfældet for en ren PL/SQL-funktion.
Vi kan demonstrere dette ved at skabe en stor funktion (SQL-kontekst) og procedure (PL/SQL-kontekst), der accepterer mange tidsstempelparametre, og derefter sammenligne inputparametrene for eventuelle forskelle. Jeg brugte tidsstempler i stedet for datoer, fordi de burde fungere på samme måde med hensyn til konsistens, men timestamp(9) er en milliard gange større sandsynlighed for at ændre sig end en dato. (Nogenlunde - der er sikkert masser af afrunding og interne urdetaljer, jeg ikke er klar over, som gør det mere kompliceret.)
--First, use functions like these to create long strings of variables.
--PL/SQL can go to 64K parameters, but luckily differences appear much sooner.
--Parameters for function and procedure
select listagg('a'||level||' timestamp', ',') within group(order by level)
from dual connect by level <= 100 order by level;
--Columns for least/greatest
select listagg('a'||level, ',') within group(order by level)
from dual connect by level <= 100;
--Systimestamps for select/exec
select listagg('systimestamp',',') within group(order by 1)
from dual connect by level <= 100;
--FUNCTION - SQL context
create or replace function function_test(
a1 timestamp,a2 timestamp,a3 timestamp,a4 timestamp,a5 timestamp,a6 timestamp,a7 timestamp,a8 timestamp,a9 timestamp,a10 timestamp,a11 timestamp,a12 timestamp,a13 timestamp,a14 timestamp,a15 timestamp,a16 timestamp,a17 timestamp,a18 timestamp,a19 timestamp,a20 timestamp,a21 timestamp,a22 timestamp,a23 timestamp,a24 timestamp,a25 timestamp,a26 timestamp,a27 timestamp,a28 timestamp,a29 timestamp,a30 timestamp,a31 timestamp,a32 timestamp,a33 timestamp,a34 timestamp,a35 timestamp,a36 timestamp,a37 timestamp,a38 timestamp,a39 timestamp,a40 timestamp,a41 timestamp,a42 timestamp,a43 timestamp,a44 timestamp,a45 timestamp,a46 timestamp,a47 timestamp,a48 timestamp,a49 timestamp,a50 timestamp,a51 timestamp,a52 timestamp,a53 timestamp,a54 timestamp,a55 timestamp,a56 timestamp,a57 timestamp,a58 timestamp,a59 timestamp,a60 timestamp,a61 timestamp,a62 timestamp,a63 timestamp,a64 timestamp,a65 timestamp,a66 timestamp,a67 timestamp,a68 timestamp,a69 timestamp,a70 timestamp,a71 timestamp,a72 timestamp,a73 timestamp,a74 timestamp,a75 timestamp,a76 timestamp,a77 timestamp,a78 timestamp,a79 timestamp,a80 timestamp,a81 timestamp,a82 timestamp,a83 timestamp,a84 timestamp,a85 timestamp,a86 timestamp,a87 timestamp,a88 timestamp,a89 timestamp,a90 timestamp,a91 timestamp,a92 timestamp,a93 timestamp,a94 timestamp,a95 timestamp,a96 timestamp,a97 timestamp,a98 timestamp,a99 timestamp,a100 timestamp
) return varchar2 is
v_min timestamp(9);
v_max timestamp(9);
begin
v_min := least(a1,a2,a3,a4,a5,a6,a7,a8,a9,a10,a11,a12,a13,a14,a15,a16,a17,a18,a19,a20,a21,a22,a23,a24,a25,a26,a27,a28,a29,a30,a31,a32,a33,a34,a35,a36,a37,a38,a39,a40,a41,a42,a43,a44,a45,a46,a47,a48,a49,a50,a51,a52,a53,a54,a55,a56,a57,a58,a59,a60,a61,a62,a63,a64,a65,a66,a67,a68,a69,a70,a71,a72,a73,a74,a75,a76,a77,a78,a79,a80,a81,a82,a83,a84,a85,a86,a87,a88,a89,a90,a91,a92,a93,a94,a95,a96,a97,a98,a99,a100
);
v_max := greatest(a1,a2,a3,a4,a5,a6,a7,a8,a9,a10,a11,a12,a13,a14,a15,a16,a17,a18,a19,a20,a21,a22,a23,a24,a25,a26,a27,a28,a29,a30,a31,a32,a33,a34,a35,a36,a37,a38,a39,a40,a41,a42,a43,a44,a45,a46,a47,a48,a49,a50,a51,a52,a53,a54,a55,a56,a57,a58,a59,a60,a61,a62,a63,a64,a65,a66,a67,a68,a69,a70,a71,a72,a73,a74,a75,a76,a77,a78,a79,a80,a81,a82,a83,a84,a85,a86,a87,a88,a89,a90,a91,a92,a93,a94,a95,a96,a97,a98,a99,a100
);
if v_min <> v_max then
return 'different';
else
return 'the same';
end if;
end;
/
--No matter how many times you run this, you'll always get "the same".
select function_test(systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp)
from dual;
--PROCEDURE - PL/SQL context
set serveroutput on;
create or replace procedure procedure_test(
a1 timestamp,a2 timestamp,a3 timestamp,a4 timestamp,a5 timestamp,a6 timestamp,a7 timestamp,a8 timestamp,a9 timestamp,a10 timestamp,a11 timestamp,a12 timestamp,a13 timestamp,a14 timestamp,a15 timestamp,a16 timestamp,a17 timestamp,a18 timestamp,a19 timestamp,a20 timestamp,a21 timestamp,a22 timestamp,a23 timestamp,a24 timestamp,a25 timestamp,a26 timestamp,a27 timestamp,a28 timestamp,a29 timestamp,a30 timestamp,a31 timestamp,a32 timestamp,a33 timestamp,a34 timestamp,a35 timestamp,a36 timestamp,a37 timestamp,a38 timestamp,a39 timestamp,a40 timestamp,a41 timestamp,a42 timestamp,a43 timestamp,a44 timestamp,a45 timestamp,a46 timestamp,a47 timestamp,a48 timestamp,a49 timestamp,a50 timestamp,a51 timestamp,a52 timestamp,a53 timestamp,a54 timestamp,a55 timestamp,a56 timestamp,a57 timestamp,a58 timestamp,a59 timestamp,a60 timestamp,a61 timestamp,a62 timestamp,a63 timestamp,a64 timestamp,a65 timestamp,a66 timestamp,a67 timestamp,a68 timestamp,a69 timestamp,a70 timestamp,a71 timestamp,a72 timestamp,a73 timestamp,a74 timestamp,a75 timestamp,a76 timestamp,a77 timestamp,a78 timestamp,a79 timestamp,a80 timestamp,a81 timestamp,a82 timestamp,a83 timestamp,a84 timestamp,a85 timestamp,a86 timestamp,a87 timestamp,a88 timestamp,a89 timestamp,a90 timestamp,a91 timestamp,a92 timestamp,a93 timestamp,a94 timestamp,a95 timestamp,a96 timestamp,a97 timestamp,a98 timestamp,a99 timestamp,a100 timestamp
) is
v_min timestamp(9);
v_max timestamp(9);
begin
v_min := least(a1,a2,a3,a4,a5,a6,a7,a8,a9,a10,a11,a12,a13,a14,a15,a16,a17,a18,a19,a20,a21,a22,a23,a24,a25,a26,a27,a28,a29,a30,a31,a32,a33,a34,a35,a36,a37,a38,a39,a40,a41,a42,a43,a44,a45,a46,a47,a48,a49,a50,a51,a52,a53,a54,a55,a56,a57,a58,a59,a60,a61,a62,a63,a64,a65,a66,a67,a68,a69,a70,a71,a72,a73,a74,a75,a76,a77,a78,a79,a80,a81,a82,a83,a84,a85,a86,a87,a88,a89,a90,a91,a92,a93,a94,a95,a96,a97,a98,a99,a100
);
v_max := greatest(a1,a2,a3,a4,a5,a6,a7,a8,a9,a10,a11,a12,a13,a14,a15,a16,a17,a18,a19,a20,a21,a22,a23,a24,a25,a26,a27,a28,a29,a30,a31,a32,a33,a34,a35,a36,a37,a38,a39,a40,a41,a42,a43,a44,a45,a46,a47,a48,a49,a50,a51,a52,a53,a54,a55,a56,a57,a58,a59,a60,a61,a62,a63,a64,a65,a66,a67,a68,a69,a70,a71,a72,a73,a74,a75,a76,a77,a78,a79,a80,a81,a82,a83,a84,a85,a86,a87,a88,a89,a90,a91,a92,a93,a94,a95,a96,a97,a98,a99,a100
);
if v_min <> v_max then
dbms_output.put_line('different');
else
dbms_output.put_line('the same');
end if;
end;
/
--I see "different" about 50% of the time.
--The ratio of differences decreases as you decrease the number of parameters.
exec procedure_test(systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp,systimestamp);