1.基本介紹
Oracle和plsql都支持lob(large object) 類型,用來存儲大數量數據,如圖像文件,聲音文件等。Oracle 9i realse2支持存儲最大爲4g的數據,oracle 10g realse1支持最大8到128萬億字節的數據存儲,依賴於你的db的block size。
在plsql中可以申明的lob類型的變量如下:
- BFILE 二進制文件,存儲在數據庫外的操作系統文件,只讀的。把此文件當二進制處理。
- BLOB 二進制大對象。存儲在數據庫裏的大對象,一般是圖像聲音等文件。
- CLOB 字符型大對象。一般存儲大數量文本信息。存儲單字節,固定寬度的數據。
- NCLOB 字節字符大對象。存儲單字節大塊,多字節固定寬度,多字節變寬度數據。
Oracle將lob分類爲兩種:
- 存儲在數據庫裏的,參與數據庫的事務。BLOB,CLOB,NCCLOB。
- 存儲在數據庫外的BFILE,不參與數據庫的事務,也就是不能rollback或commit等,它依賴於文件系統的數據完整性。
RAW這兩種數據類型也是存儲字符的,但是有系列的問題,不建議使用,這裏也就不討論了。
2.LOB的使用
本部分使用的表是:
- /**
- table script
- **/
- CREATE TABLE waterfalls (
- falls_name VARCHAR2(80),--name
- falls_photo BLOB,--照片
- falls_directions CLOB,--文字
- falls_description NCLOB,--文字
- falls_web_page BFILE);--指向外部的html頁面
- /
- /**
- table script
- **/
- CREATE TABLE waterfalls (
- falls_name VARCHAR2(80),--name
- falls_photo BLOB,--照片
- falls_directions CLOB,--文字
- falls_description NCLOB,--文字
- falls_web_page BFILE);--指向外部的html頁面
- /
1. 理解LOB的Locator
在plsql中申明瞭一個lob類型的變量,然後從數據庫中查詢一個lob類型的值分配給變量,也只是將指針複製給了它,那麼這個變量也會指向數據庫中實際存放lob數據的地方。如:
- --understanding lob locators
- DECLARE
- photo BLOB;
- BEGIN
- SELECT falls_photo
- INTO photo
- FROM waterfalls
- WHERE falls_name='Dryer Hose';
- --understanding lob locators
- DECLARE
- photo BLOB;
- BEGIN
- SELECT falls_photo
- INTO photo
- FROM waterfalls
- WHERE falls_name='Dryer Hose';
從上面的圖可以看出,要處理lob數據,必須先獲得lob locators。我們可以通過一個select語句獲取,當賦值給lob變量的時候,它也獲得同樣的lob locators。我們在plsql中處理可以使用dbms_lob包,裏面內置了很多過程和函數來讀取和修改我們的lob數據。下面給出處理lob數據的一般方法。
1. 通過select語句獲取一個lob locator。
2. 通過調用dbms_lob.open打開lob。
3. 調用dbms_lob.getchunksize獲得最佳讀寫lob值。
4. 調用dbms_lob.getlength獲取lob數據的字節值。
5. 調用dbms_lob.read獲取lob數據。
6. 調用dbms_lob.close關閉lob。
2.Empty
lob and Null lob
locator,但是沒有指向任何lob數據。Null是定義了一個變量,但是沒有獲得lob locator。對lob類型的處理和其他類型不一樣。如下面的例子:
- /* null lob example*/
- DECLARE
- directions CLOB; --定義了,但是沒有分配值,爲null
- BEGIN
- IF directions IS NULL THEN
- dbms_output.put_line('directions is null');
- ELSE
- dbms_output.put_line('directions is not null');
- END IF;
- END;
- /
- DECLARE
- directions CLOB; --定義一個,並且分配值
- BEGIN
- --刪除一行
- DELETE FROM waterfalls WHERE falls_name = 'Munising Falls';
- --插入一行通過使用 EMPTY_CLOB( ) to 建立一個lob locator
- INSERT INTO waterfalls
- (falls_name, falls_directions)
- VALUES
- ('Munising Falls', empty_clob());
- --獲得lob locator,上面插入的數據,因爲我們插入的是一個empty_clob(),那麼lob locator不指向任何數據,雖然給變量分配了只
- SELECT falls_directions
- INTO directions
- FROM waterfalls
- WHERE falls_name = 'Munising Falls';
- IF directions IS NULL THEN
- dbms_output.put_line('directions is NULL');
- ELSE
- dbms_output.put_line('directions is not NULL'); --打印此句
- END IF;
- dbms_output.put_line('Length = ' || dbms_lob.getlength(directions)); --結果爲o
- END;
- /* null lob example*/
- DECLARE
- directions CLOB; --定義了,但是沒有分配值,爲null
- BEGIN
- IF directions IS NULL THEN
- dbms_output.put_line('directions is null');
- ELSE
- dbms_output.put_line('directions is not null');
- END IF;
- END;
- /
- DECLARE
- directions CLOB; --定義一個,並且分配值
- BEGIN
- --刪除一行
- DELETE FROM waterfalls WHERE falls_name = 'Munising Falls';
- --插入一行通過使用 EMPTY_CLOB( ) to 建立一個lob locator
- INSERT INTO waterfalls
- (falls_name, falls_directions)
- VALUES
- ('Munising Falls', empty_clob());
- --獲得lob locator,上面插入的數據,因爲我們插入的是一個empty_clob(),那麼lob locator不指向任何數據,雖然給變量分配了只
- SELECT falls_directions
- INTO directions
- FROM waterfalls
- WHERE falls_name = 'Munising Falls';
- IF directions IS NULL THEN
- dbms_output.put_line('directions is NULL');
- ELSE
- dbms_output.put_line('directions is not NULL'); --打印此句
- END IF;
- dbms_output.put_line('Length = ' || dbms_lob.getlength(directions)); --結果爲o
- END;
- 上面例子中的empty_clob()是oracle的內置函數,創建了一個lob locator。但是我們沒有讓它指向任何數據,所以是empty。而且通過select語句給變量directions分配了lob locator,所以不是null,但是length爲0,故爲empty。
- 在基本類型中,我們判斷一個變量是不是有數據,只要is null就可以了。但是在lob類型中我們從以上的例子看出來是不正確的。Lob首先必須判斷is null看是否分配lob locator,如果分配了還需要進一步檢查length是否爲0,看是否是empty,所以完整的是下面這樣:
- IF some_clob IS NULL THEN
- --如果is null爲true表示未分配,肯定沒有數據
- ELSIF dbms_lob.getlength(some_clob) = 0 THEN
- --分配了length爲0,也沒有數據
- ELSE
- --有數據
- END IF;
- IF some_clob IS NULL THEN
- --如果is null爲true表示未分配,肯定沒有數據
- ELSIF dbms_lob.getlength(some_clob) = 0 THEN
- --分配了length爲0,也沒有數據
- ELSE
- --有數據
- END IF;
3.建立LOB
locator只是一個指針,真正的數據是存儲在磁盤中或數據庫文件中。我 們先建立一個空的clob,然後我們可以update來讓變量真正指向有數據的lob。Empty_clob()可以用來處理clob和nclob。在oracle 8i中可以使用temporary lob達到同樣的效果。
4.
向LOB裏寫入數據
locator之後,就可以使用dbms_lob包的下列procedure向lob中寫入數據。
DBMS_LOB.WRITE:允許自動寫入數據到lob中。
DBMS_LOB.WRITEAPPEND:向lob的末尾寫入數據。
- --write lob
- DECLARE
- directions CLOB;
- amount BINARY_INTEGER;
- offset INTEGER;
- first_direction VARCHAR2(100);
- more_directions VARCHAR2(500);
- BEGIN
- --Delete any existing rows for 'Munising Falls' so that this
- --example can be executed multiple times
- DELETE FROM waterfalls WHERE falls_name = 'Munising Falls';
- --Insert a new row using EMPTY_CLOB( ) to create a LOB locator
- INSERT INTO waterfalls
- (falls_name, falls_directions)
- VALUES
- ('Munising Falls', empty_clob());
- --Retrieve the LOB locator created by the previous INSERT statement
- SELECT falls_directions
- INTO directions
- FROM waterfalls
- WHERE falls_name = 'Munising Falls';
- --Open the LOB; not strictly necessary, but best to open/close LOBs.
- dbms_lob.open(directions, dbms_lob.lob_readwrite);
- --Use DBMS_LOB.WRITE to begin
- first_direction := 'Follow I-75 across the Mackinac Bridge.';
- amount := length(first_direction); --number of characters to write
- offset := 1; --begin writing to the first character of the CLOB
- dbms_lob.write(directions, amount, offset, first_direction);
- --Add some more directions using DBMS_LOB.WRITEAPPEND
- more_directions := ' Take US-2 west from St. Ignace to Blaney Park.' ||
- ' Turn north on M-77 and drive to Seney.' ||
- ' From Seney, take M-28 west to Munising.';
- dbms_lob.writeappend(directions,
- length(more_directions),
- more_directions);
- --Add yet more directions
- more_directions := ' In front of the paper mill, turn right on H-58.' ||
- ' Follow H-58 to Washington Street. Veer left onto' ||
- ' Washington Street. You''ll find the Munising' ||
- ' Falls visitor center across from the hospital at' ||
- ' the point where Washington Street becomes' ||
- ' Sand Point Road.';
- dbms_lob.writeappend(directions,
- length(more_directions),
- more_directions);
- --Close the LOB, and we are done.
- dbms_lob.close(directions);
- END;
- --write lob
- DECLARE
- directions CLOB;
- amount BINARY_INTEGER;
- offset INTEGER;
- first_direction VARCHAR2(100);
- more_directions VARCHAR2(500);
- BEGIN
- --Delete any existing rows for 'Munising Falls' so that this
- --example can be executed multiple times
- DELETE FROM waterfalls WHERE falls_name = 'Munising Falls';
- --Insert a new row using EMPTY_CLOB( ) to create a LOB locator
- INSERT INTO waterfalls
- (falls_name, falls_directions)
- VALUES
- ('Munising Falls', empty_clob());
- --Retrieve the LOB locator created by the previous INSERT statement
- SELECT falls_directions
- INTO directions
- FROM waterfalls
- WHERE falls_name = 'Munising Falls';
- --Open the LOB; not strictly necessary, but best to open/close LOBs.
- dbms_lob.open(directions, dbms_lob.lob_readwrite);
- --Use DBMS_LOB.WRITE to begin
- first_direction := 'Follow I-75 across the Mackinac Bridge.';
- amount := length(first_direction); --number of characters to write
- offset := 1; --begin writing to the first character of the CLOB
- dbms_lob.write(directions, amount, offset, first_direction);
- --Add some more directions using DBMS_LOB.WRITEAPPEND
- more_directions := ' Take US-2 west from St. Ignace to Blaney Park.' ||
- ' Turn north on M-77 and drive to Seney.' ||
- ' From Seney, take M-28 west to Munising.';
- dbms_lob.writeappend(directions,
- length(more_directions),
- more_directions);
- --Add yet more directions
- more_directions := ' In front of the paper mill, turn right on H-58.' ||
- ' Follow H-58 to Washington Street. Veer left onto' ||
- ' Washington Street. You''ll find the Munising' ||
- ' Falls visitor center across from the hospital at' ||
- ' the point where Washington Street becomes' ||
- ' Sand Point Road.';
- dbms_lob.writeappend(directions,
- length(more_directions),
- more_directions);
- --Close the LOB, and we are done.
- dbms_lob.close(directions);
- END;
我們向lob中寫入數據的時候,沒有必要更新表中的列。因爲它保存的只是一個locator,我們的變量也獲得同樣的locator,當我們寫入數據去lob的時候,locator並沒有改變。改變的只是locator指向的物理數據。
5.
從lob中讀取數據
下面是dbms_lob.read過程的定義,注意參數.
- PROCEDURE READ(lob_loc IN BLOB, --初始化後的lob變量lob locator
- amount IN OUT NOCOPY INTEGER, --讀取的數量(clob爲字符數,blob,bfile是字節數)
- offset IN INTEGER, --開始讀取位置
- buffer OUT RAW); --讀到的數據,raw要顯示用轉換函數,見bfile
- PROCEDURE READ(lob_loc IN CLOB CHARACTER SET any_cs,
- amount IN OUT NOCOPY INTEGER,
- offset IN INTEGER,
- buffer OUT VARCHAR2 CHARACTER SET lob_loc%charset);
- PROCEDURE READ(file_loc IN BFILE,
- amount IN OUT NOCOPY INTEGER,
- offset IN INTEGER,
- buffer OUT RAW);
- PROCEDURE READ(lob_loc IN BLOB, --初始化後的lob變量lob locator
- amount IN OUT NOCOPY INTEGER, --讀取的數量(clob爲字符數,blob,bfile是字節數)
- offset IN INTEGER, --開始讀取位置
- buffer OUT RAW); --讀到的數據,raw要顯示用轉換函數,見bfile
- PROCEDURE READ(lob_loc IN CLOB CHARACTER SET any_cs,
- amount IN OUT NOCOPY INTEGER,
- offset IN INTEGER,
- buffer OUT VARCHAR2 CHARACTER SET lob_loc%charset);
- PROCEDURE READ(file_loc IN BFILE,
- amount IN OUT NOCOPY INTEGER,
- offset IN INTEGER,
- buffer OUT RAW);
- --從lob中讀取數據
- DECLARE
- directions CLOB;
- directions_1 VARCHAR2(300);
- directions_2 VARCHAR2(300);
- chars_read_1 BINARY_INTEGER;
- chars_read_2 BINARY_INTEGER;
- offset INTEGER;
- BEGIN
- --首先獲得一個lob locator
- SELECT falls_directions
- INTO directions
- FROM waterfalls
- WHERE falls_name = 'Munising Falls';
- --記錄開始讀取位置
- offset := 1;
- --嘗試讀取229個字符,chars_read_1將被實際讀取的字符數更新
- chars_read_1 := 229;
- dbms_lob.read(directions, chars_read_1, offset, directions_1);
- --當讀取229個字符之後,更新offset,再讀取225個字符
- IF chars_read_1 = 229 THEN
- offset := offset + chars_read_1; --offset變爲offset+chars_read_1,也就是從300開始
- chars_read_2 := 255;
- dbms_lob.read(directions, chars_read_2, offset, directions_2);
- ELSE
- chars_read_2 := 0; --否則後面不在讀取
- directions_2 := '';
- END IF;
- --顯示讀取的字符數
- dbms_output.put_line('Characters read = ' ||
- to_char(chars_read_1 + chars_read_2));
- --顯示結果
- dbms_output.put_line(directions_1);
- dbms_output.put_line(length(directions_1));
- dbms_output.put_line(directions_2);
- dbms_output.put_line(length(directions_2));
- END;
- --從lob中讀取數據
- DECLARE
- directions CLOB;
- directions_1 VARCHAR2(300);
- directions_2 VARCHAR2(300);
- chars_read_1 BINARY_INTEGER;
- chars_read_2 BINARY_INTEGER;
- offset INTEGER;
- BEGIN
- --首先獲得一個lob locator
- SELECT falls_directions
- INTO directions
- FROM waterfalls
- WHERE falls_name = 'Munising Falls';
- --記錄開始讀取位置
- offset := 1;
- --嘗試讀取229個字符,chars_read_1將被實際讀取的字符數更新
- chars_read_1 := 229;
- dbms_lob.read(directions, chars_read_1, offset, directions_1);
- --當讀取229個字符之後,更新offset,再讀取225個字符
- IF chars_read_1 = 229 THEN
- offset := offset + chars_read_1; --offset變爲offset+chars_read_1,也就是從300開始
- chars_read_2 := 255;
- dbms_lob.read(directions, chars_read_2, offset, directions_2);
- ELSE
- chars_read_2 := 0; --否則後面不在讀取
- directions_2 := '';
- END IF;
- --顯示讀取的字符數
- dbms_output.put_line('Characters read = ' ||
- to_char(chars_read_1 + chars_read_2));
- --顯示結果
- dbms_output.put_line(directions_1);
- dbms_output.put_line(length(directions_1));
- dbms_output.put_line(directions_2);
- dbms_output.put_line(length(directions_2));
- END;
Dbms_lob.read的第2個參數是傳遞要讀取的數量。對於clob是字符數,blob和bfile都是字節數。它是隨着讀取的數目自動更新的,offset不會更新。所以分佈讀取需要手動更新offset,下個offset是上一個offset+讀取的數量。我們可以通過dbms_lob.get_length(lob_locator)獲得這個lob的長度,結果clob是字符數,blob和bfile是字節數,然後分佈讀取。