是否可以編輯或修改內置的MySQL函數？

Question

是否有可能改變/修改MySQL的建成使用MySQL命令行功能，例如date()功能

或者，我們可以看到實際的功能是怎麼寫的？

編輯實際原因

的原因是我有一個自定義的書面MySQL的函數像下面

CREATE FUNCTION getUser() RETURNS char(50) CHARSET latin1 
RETURN @empNumber; 

但是，它是有點慢。我想看看如何編寫實際的內置函數，然後我可以按照相同的步驟。

Answer 1

如果你有一種編程語言允許你編寫一個DLL，你可以創建一個自定義的UDF。 http://www.codeguru.com/cpp/data/mfc_database/misc/article.php/c12615

這裏使用德爾福的例子：：http://www.sherlocksoftware.org/page.php?id=58

下面是一個萊文斯坦功能樣本源代碼（德爾福）：

unit udf_levenshtein; 

interface 

{Nicholas Sherlock, 2006. 

Example of a UDF: Levenshtein string distance. 
After copying the dll to your MySql/bin folder, create with: 
CREATE FUNCTION Levenshtein RETURNS INT SONAME 'LevenshteinUDF.dll';} 

uses SysUtils, mysql_udf; 

function stringdistance(const s, t: string): integer; overload; 
function stringdistance(const s, t: string; maxdist: integer; out dist: integer): boolean; overload; 

implementation 

type 
    TIntArray = array[0..maxint div 4 - 1] of integer; {Avoid errors if range check on} 
    pintarray = ^TIntArray; 

function Levenshtein_init(var init: UDF_INIT; var _args: UDF_ARGS; message: pChar): my_bool; cdecl; 
var args: TUDF_Args; 
begin 
    result := 0; 
    args := TUDF_Args.create(@_args); 
    try 
    if (not (args.Count in [2..3])) or ((args.count = 3) and (args[2].ArgType <> INT_RESULT)) then begin 
     strcopy(message, 'Levenshtein() usage: Levenshtein(string1, string2 [,maxdist])'); 
     result := 1; //fail 
     exit; 
    end; 

    args[0].ArgType := STRING_RESULT; 
    args[1].ArgType := STRING_RESULT; //Mysql will now coerce these args to string for us 

    if args.count = 3 then 
     init.maybe_null := 1 else //since we could exceed maxdist 
     init.maybe_null := 0; 

    finally 
    args.free; 
    end; 
end; 

procedure Levenshtein_deinit(var init: UDF_INIT); cdecl; 
begin 
{Do nothing. We could actually omit this function, MySql won't 
    try to call it if it doesn't exist} 
end; 

function Levenshtein(var init: UDF_INIT; var _args: UDF_ARGS; var is_null, error: byte): int64; cdecl; 
var args: TUDF_Args; 
    dist: integer; 
begin 
    error := 0; 
    is_null := 0; 
    args := TUDF_Args.create(@_args); 
    try 
    if args.Count >= 3 then begin 
     if not stringdistance(args[0].DataString, args[1].DataString, args[2].DataInteger, dist) then begin 
     result := 0; 
     is_null := 1; 
     end else //didn't fall inside maximum distance 
     result := dist; 
    end else 
     result := stringdistance(args[0].DataString, args[1].DataString); 
    finally 
    args.free; 
    end; 
end; 

function min3(a, b, c: integer): integer; 
begin 
    if (a <= b) and (a <= c) then begin 
    result := a; 
    exit; 
    end; 
    if (b <= a) and (b <= c) then begin 
    result := b; 
    Exit; 
    end; 
    result := c; 
end; 

{Find the distance between the two strings s and t. We are only interested in 
the distance if the distance is less than or equal to maxdist. The result is 
true if the distance has been found (Distance stored in dist), it may be greater 
than maxdist. If the result is false, then the distance has not been found, but 
it is greater than maxdist} 

function stringdistance(const s, t: string; maxdist: integer; out dist: integer): boolean; 
const 
    cost_ins = 1; 
    cost_del = 1; 
    cost_sub = 1; 
var n, m: integer; 
    p, q, r: PIntArray; 
    smallest, cost, i, j: integer; 
begin 
    result := false; //Didn't meet max dist requirement 

    m := length(s); 
    n := length(t); 

    if abs(m - n) > maxdist then 
    exit; //dist is always at least the difference between lengths of args 

    if n = 0 then begin 
    dist := m; 
    result := true; 
    exit; 
    end else 
    if m = 0 then begin 
     dist := n; 
     result := true; 
     exit; 
    end; 

    getmem(p, (n + 1) * SizeOf(integer)); {p is first row} 
    getmem(q, (n + 1) * SizeOf(integer)); {q is second row} 

    p[0] := 0; 
    for i := 1 to n do 
    p[i] := p[i - 1] + cost_ins; {fill first row} 

    for i := 1 to m do begin 
    q[0] := p[0] + cost_del; //cost del 
    smallest := q[0]; 
    for j := 1 to n do begin 
     if s[i] = t[j] then cost := 0 else cost := cost_sub; 
     q[j] := min3(p[j] + cost_del, 
     q[j - 1] + cost_ins, 
     p[j - 1] + cost {cost subst} 
     ); 
     if q[j]<smallest then 
     smallest:=q[j]; 
    end; 
    if smallest > maxdist then 
     exit; //exceeded maxdist 

    r := p; {Swap over the two rows} 
    p := q; 
    q := r; 
    end; 

    dist := p[n]; 
    result := true; 

    FreeMem(p); 
    FreeMem(q); 
end; 

function stringdistance(const s, t: string): integer; 
const 
    cost_ins = 1; 
    cost_del = 1; 
    cost_sub = 1; 
var n, m: integer; 
    p, q, r: PIntArray; 
    cost, i, j: integer; 
begin 
    m := length(s); 
    n := length(t); 

    if n = 0 then begin 
    result := m; 
    exit; 
    end else 
    if m = 0 then begin 
     result := n; 
     exit; 
    end; 

    getmem(p, (n + 1) * SizeOf(integer)); {p is first row} 
    getmem(q, (n + 1) * SizeOf(integer)); {q is second row} 

    p[0] := 0; 
    for i := 1 to n do 
    p[i] := p[i - 1] + cost_ins; {fill first row} 

    for i := 1 to m do begin 
    q[0] := p[0] + cost_del; 
    for j := 1 to n do begin 
     if s[i] = t[j] then cost := 0 else cost := cost_sub; 
     q[j] := min3(p[j] + cost_del, {cost del} 
     q[j - 1] + cost_ins, {cost ins} 
     p[j - 1] + cost {cost subst} 
     ); 
    end; 

    r := p; {Swap over the two rows} 
    p := q; 
    q := r; 
    end; 

    result := p[n]; 

    FreeMem(p); 
    FreeMem(q); 
end; 

exports 
    Levenshtein_init, 
    Levenshtein, 
    Levenshtein_deinit; 

end.