基於MySQL自增ID欄位增量掃描研究
目錄
1. 問題
對於MySQL表,如果自增ID不是主鍵時,是否可以用來做增量查詢?
2. 背景
需要按照自增ID欄位進行增量查詢,有些表的自增ID是主鍵,而有些表的自增只是普通索引,有些採用MyISAM,有些採用InnoDB。
如果採用粗暴的“SELECT * FROM table WHERE f_id>M ORDER BY f_id LIMIT N”,功能上沒有任何問題,但當表的記錄數很大時(比如1000萬條),“ORDER BY f_id”會極影響查詢效率。為此,需要弄清楚“SELECT * FROM table WHERE f_id>M LIMIT N”的可行性,即增量查詢時,不指定“ORDER BY f_id”。
研究基於的MySQL(注:5.6.7之前最大分割槽數限制為1024,從5.6.7開始調整為8192,另外5.6版本分割槽表不支援HANDLER):
MySQL [test]> select version(); +-----------+ | version() | +-----------+ | 5.7.18 | +-----------+ 1 row in set (0.01 sec) |
3. InnoDB表
3.1. 自增ID為主鍵
建表SQL語句:
DROP TABLE IF EXISTS `tableA1`; CREATE TABLE `tableA1` ( `id` BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY, `af` INT NOT NULL, `bf` INT NOT NULL, `cf` INT NOT NULL, INDEX `idx_af` (`af`), INDEX `idx_bf` (`bf`) )ENGINE=InnoDB; |
依順序執行下列插入操作:
INSERT INTO tableA1 (af,bf,cf) VALUES (1,2,1); INSERT INTO tableA1 (af,bf,cf) VALUES (2,1,2); INSERT INTO tableA1 (id,af,bf,cf) VALUES (11,12,11,11); INSERT INTO tableA1 (id,af,bf,cf) VALUES (12,11,12,12); INSERT INTO tableA1 (af,bf,cf) VALUES (13,16,13); INSERT INTO tableA1 (id,af,bf,cf) VALUES (3,3,3,3); INSERT INTO tableA1 (af,bf,cf) VALUES (14,17,14); INSERT INTO tableA1 (id,af,bf,cf) VALUES (5,15,5,5); |
檢視結果:
// 按自增ID有序(自增ID為主鍵) MySQL [test]> SELECT * FROM tableA1; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 2 | 1 | | 2 | 2 | 1 | 2 | | 3 | 3 | 3 | 3 | | 5 | 15 | 5 | 5 | | 11 | 12 | 11 | 11 | | 12 | 11 | 12 | 12 | | 13 | 13 | 16 | 13 | | 14 | 14 | 17 | 14 | +----+----+----+----+ 8 rows in set (0.00 sec) // 按自增ID有序(自增ID為主鍵) MySQL [test]> SELECT * FROM tableA1 WHERE id>=1 LIMIT 10; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 2 | 1 | | 2 | 2 | 1 | 2 | | 3 | 3 | 3 | 3 | | 5 | 15 | 5 | 5 | | 11 | 12 | 11 | 11 | | 12 | 11 | 12 | 12 | | 13 | 13 | 16 | 13 | | 14 | 14 | 17 | 14 | +----+----+----+----+ 8 rows in set (0.00 sec)
// 按自增ID有序(自增ID為主鍵) MySQL [test]> SELECT * FROM tableA1 WHERE id>=2 LIMIT 10; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 2 | 2 | 1 | 2 | | 3 | 3 | 3 | 3 | | 5 | 15 | 5 | 5 | | 11 | 12 | 11 | 11 | | 12 | 11 | 12 | 12 | | 13 | 13 | 16 | 13 | | 14 | 14 | 17 | 14 | +----+----+----+----+ 7 rows in set (0.00 sec)
// 按自增ID有序(自增ID為主鍵) MySQL [test]> SELECT * FROM tableA1 WHERE id>=2 LIMIT 7; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 2 | 2 | 1 | 2 | | 3 | 3 | 3 | 3 | | 5 | 15 | 5 | 5 | | 11 | 12 | 11 | 11 | | 12 | 11 | 12 | 12 | | 13 | 13 | 16 | 13 | | 14 | 14 | 17 | 14 | +----+----+----+----+ 7 rows in set (0.00 sec) |
可以看到,當自增ID為主鍵時,自增ID亂序插入,查詢結果也是按自增ID有序(實測有序插入一樣有序),因此可以放心依自增ID增量查詢,而不必指定“ORDER BY f_id”。
3.2. 自增ID為普通索引
DROP TABLE IF EXISTS `tableA2`; CREATE TABLE `tableA2` ( `id` BIGINT NOT NULL AUTO_INCREMENT, `af` INT NOT NULL, `bf` INT NOT NULL, `cf` INT NOT NULL, UNIQUE INDEX `idx_af` (`af`), INDEX `idx_id` (`id`), INDEX `idx_bf` (`bf`) )ENGINE=InnoDB; |
依順序執行下列插入操作:
INSERT INTO tableA2 (af,bf,cf) VALUES (1,2,1); INSERT INTO tableA2 (af,bf,cf) VALUES (2,1,2); INSERT INTO tableA2 (id,af,bf,cf) VALUES (11,12,11,11); INSERT INTO tableA2 (id,af,bf,cf) VALUES (12,11,12,12); INSERT INTO tableA2 (af,bf,cf) VALUES (13,16,13); INSERT INTO tableA2 (id,af,bf,cf) VALUES (3,3,3,3); INSERT INTO tableA2 (af,bf,cf) VALUES (14,17,14); INSERT INTO tableA2 (id,af,bf,cf) VALUES (5,15,5,5); |
檢視結果:
// 總共8條記錄 MySQL [test]> SELECT COUNT(1) FROM tableA2; +----------+ | COUNT(1) | +----------+ | 8 | +----------+ 1 row in set (0.00 sec)
// 按自增ID無序,但按唯一索引有序 MySQL [test]> SELECT * FROM tableA2; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 2 | 1 | | 2 | 2 | 1 | 2 | | 3 | 3 | 3 | 3 | | 12 | 11 | 12 | 12 | | 11 | 12 | 11 | 11 | | 13 | 13 | 16 | 13 | | 14 | 14 | 17 | 14 | | 5 | 15 | 5 | 5 | +----+----+----+----+ 8 rows in set (0.00 sec)
// 按自增ID無序,但按唯一索引有序 MySQL [test]> SELECT * FROM tableA2 WHERE id>=1 LIMIT 10; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 2 | 1 | | 2 | 2 | 1 | 2 | | 3 | 3 | 3 | 3 | | 12 | 11 | 12 | 12 | | 11 | 12 | 11 | 11 | | 13 | 13 | 16 | 13 | | 14 | 14 | 17 | 14 | | 5 | 15 | 5 | 5 | +----+----+----+----+ 8 rows in set (0.00 sec)
// 按自增ID無序,但按唯一索引有序 MySQL [test]> SELECT * FROM tableA2 WHERE id>=2 LIMIT 10; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 2 | 2 | 1 | 2 | | 3 | 3 | 3 | 3 | | 12 | 11 | 12 | 12 | | 11 | 12 | 11 | 11 | | 13 | 13 | 16 | 13 | | 14 | 14 | 17 | 14 | | 5 | 15 | 5 | 5 | +----+----+----+----+ 7 rows in set (0.00 sec)
// 按自增ID有序,但按唯一索引無序(LIMIT數小於表總記錄數) MySQL [test]> SELECT * FROM tableA2 WHERE id>=2 LIMIT 5; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 2 | 2 | 1 | 2 | | 3 | 3 | 3 | 3 | | 5 | 15 | 5 | 5 | | 11 | 12 | 11 | 11 | | 12 | 11 | 12 | 12 | +----+----+----+----+ 5 rows in set (0.00 sec)
// 按自增ID有序,但按唯一索引無序(LIMIT數小於表總記錄數) MySQL [test]> SELECT * FROM tableA2 WHERE id>=1 LIMIT 7; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 2 | 1 | | 2 | 2 | 1 | 2 | | 3 | 3 | 3 | 3 | | 5 | 15 | 5 | 5 | | 11 | 12 | 11 | 11 | | 12 | 11 | 12 | 12 | | 13 | 13 | 16 | 13 | +----+----+----+----+ 7 rows in set (0.00 sec)
// 更新一條記錄 MySQL [test]> UPDATE tableA2 SET id=15 WHERE id=12; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0
// 按自增ID是無序的 MySQL [test]> SELECT * FROM tableA2 LIMIT 7; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 2 | 1 | | 2 | 2 | 1 | 2 | | 3 | 3 | 3 | 3 | | 15 | 11 | 12 | 12 | | 11 | 12 | 11 | 11 | | 13 | 13 | 16 | 13 | | 14 | 14 | 17 | 14 | +----+----+----+----+ 7 rows in set (0.00 sec) // 按自增ID是有序的(LIMIT數小於表記錄數) // 按唯一自增ID無序 MySQL [test]> SELECT * FROM tableA2 WHERE id>=1 LIMIT 7; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 2 | 1 | | 2 | 2 | 1 | 2 | | 3 | 3 | 3 | 3 | | 5 | 15 | 5 | 5 | | 11 | 12 | 11 | 11 | | 13 | 13 | 16 | 13 | | 14 | 14 | 17 | 14 | +----+----+----+----+ 7 rows in set (0.00 sec)
// 按自增ID是無序的(LIMIT數等於或大於表記錄數) // 按唯一自增ID有序 MySQL [test]> SELECT * FROM tableA2 WHERE id>=1 LIMIT 8; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 2 | 1 | | 2 | 2 | 1 | 2 | | 3 | 3 | 3 | 3 | | 15 | 11 | 12 | 12 | | 11 | 12 | 11 | 11 | | 13 | 13 | 16 | 13 | | 14 | 14 | 17 | 14 | | 5 | 15 | 5 | 5 | +----+----+----+----+ 8 rows in set (0.00 sec) |
從測試可以看到,當LIMIT的數小於表的記錄數時,結果是按自增ID有序返回。
3.3. 原因分析
InnoDB儲存資料時,即按B+樹結果儲存,B+樹的葉子結果儲存完整的記錄,表檔案本身即為主索引(即主鍵),普通索引並不直接指向資料,而是指向了主索引。
如對於表tableA2(tableA1結果相同):
INSERT INTO tableA2 (id,af,bf,cf) VALUES (1,1,1,1); INSERT INTO tableA2 (id,af,bf,cf) VALUES (2,5,1,1); INSERT INTO tableA2 (id,af,bf,cf) VALUES (3,2,1,1); INSERT INTO tableA2 (id,af,bf,cf) VALUES (4,8,1,1); INSERT INTO tableA2 (id,af,bf,cf) VALUES (5,3,1,1); INSERT INTO tableA2 (id,af,bf,cf) VALUES (6,4,1,1); INSERT INTO tableA2 (id,af,bf,cf) VALUES (7,7,1,1);
MySQL [test]> SELECT * FROM tableA2 WHERE af>0 LIMIT 3; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 1 | 1 | | 3 | 2 | 1 | 1 | | 5 | 3 | 1 | 1 | +----+----+----+----+ 3 rows in set (0.00 sec)
MySQL [test]> SELECT * FROM tableA2 WHERE af>0 LIMIT 10; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 1 | 1 | | 3 | 2 | 1 | 1 | | 5 | 3 | 1 | 1 | | 6 | 4 | 1 | 1 | | 2 | 5 | 1 | 1 | | 7 | 7 | 1 | 1 | | 4 | 8 | 1 | 1 | +----+----+----+----+ 7 rows in set (0.00 sec) |
4. MyISAM表
4.1. 自增ID為主鍵
建表SQL語句:
DROP TABLE IF EXISTS `tableB1`; CREATE TABLE `tableB1` ( `id` BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY, `af` INT NOT NULL, `bf` INT NOT NULL, `cf` INT NOT NULL, INDEX `idx_id` (`id`), INDEX `idx_bf` (`bf`) )ENGINE=MyISAM; |
依順序執行下列插入操作:
INSERT INTO tableB1 (af,bf,cf) VALUES (1,2,1); INSERT INTO tableB1 (af,bf,cf) VALUES (2,1,2); INSERT INTO tableB1 (id,af,bf,cf) VALUES (11,12,11,11); INSERT INTO tableB1 (id,af,bf,cf) VALUES (12,11,12,12); INSERT INTO tableB1 (af,bf,cf) VALUES (13,16,13); INSERT INTO tableB1 (id,af,bf,cf) VALUES (3,3,3,3); INSERT INTO tableB1 (af,bf,cf) VALUES (14,17,14); INSERT INTO tableB1 (id,af,bf,cf) VALUES (5,15,5,5); |
檢視結果:
// 亂序 MySQL [test]> SELECT * FROM tableB1; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 1 | 1 | | 2 | 2 | 2 | 2 | | 11 | 11 | 11 | 11 | | 12 | 12 | 12 | 12 | | 13 | 13 | 13 | 13 | | 3 | 3 | 3 | 3 | | 14 | 14 | 14 | 14 | | 5 | 5 | 5 | 5 | +----+----+----+----+ 8 rows in set (0.00 sec)
// 亂序了 MySQL [test]> SELECT * FROM tableB1 WHERE id>1 LIMIT 10; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 2 | 2 | 2 | 2 | | 11 | 11 | 11 | 11 | | 12 | 12 | 12 | 12 | | 13 | 13 | 13 | 13 | | 3 | 3 | 3 | 3 | | 14 | 14 | 14 | 14 | | 5 | 5 | 5 | 5 | +----+----+----+----+ 7 rows in set (0.00 sec) |
可以看到,結果並不是按自增ID有序,但是否意味著不能用來做增量查詢了?繼續看下面的操作:
MySQL [test]> SELECT COUNT(1) FROM tableB1; +----------+ | COUNT(1) | +----------+ | 8 | +----------+ 1 row in set (0.00 sec)
MySQL [test]> SELECT * FROM tableB1 WHERE id>0 LIMIT 3; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 1 | 1 | | 2 | 2 | 2 | 2 | | 3 | 3 | 3 | 3 | +----+----+----+----+ 3 rows in set (0.01 sec)
// 未亂序 MySQL [test]> SELECT * FROM tableB1 WHERE id>0 LIMIT 7; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 1 | 1 | | 2 | 2 | 2 | 2 | | 3 | 3 | 3 | 3 | | 5 | 5 | 5 | 5 | | 11 | 11 | 11 | 11 | | 12 | 12 | 12 | 12 | | 13 | 13 | 13 | 13 | +----+----+----+----+ 7 rows in set (0.00 sec)
// 亂序 MySQL [test]> SELECT * FROM tableB1 WHERE id>0 LIMIT 8; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 1 | 1 | | 2 | 2 | 2 | 2 | | 11 | 11 | 11 | 11 | | 12 | 12 | 12 | 12 | | 13 | 13 | 13 | 13 | | 3 | 3 | 3 | 3 | | 14 | 14 | 14 | 14 | | 5 | 5 | 5 | 5 | +----+----+----+----+ 8 rows in set (0.00 sec)
MySQL [jay_data]> SELECT * FROM tableB1 WHERE id>6 LIMIT 3; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 11 | 11 | 11 | 11 | | 12 | 12 | 12 | 12 | | 13 | 13 | 13 | 13 | +----+----+----+----+ 3 rows in set (0.01 sec)
MySQL [jay_data]> SELECT * FROM tableB1 WHERE id>8 LIMIT 3; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 11 | 11 | 11 | 11 | | 12 | 12 | 12 | 12 | | 13 | 13 | 13 | 13 | +----+----+----+----+ 3 rows in set (0.00 sec) |
這裡發生了有趣的事,如果LIMIT指定的數小於表實際的記錄數,則仍然是按ID有序,否則是ID是亂序的。但是實際遇到:即使LIMIT指定的數小於表實際的記錄數,也會返回亂序的結果。
4.2. 自增ID為普通索引
建表SQL語句:
DROP TABLE IF EXISTS `tableB2`; CREATE TABLE `tableB2` ( `id` BIGINT NOT NULL AUTO_INCREMENT, `af` INT NOT NULL, `bf` INT NOT NULL, `cf` INT NOT NULL, UNIQUE INDEX `idx_af` (`af`), INDEX `idx_id` (`id`), INDEX `idx_bf` (`bf`) )ENGINE=MyISAM; |
依順序執行下列插入操作:
INSERT INTO tableB2 (af,bf,cf) VALUES (1,2,1); INSERT INTO tableB2 (af,bf,cf) VALUES (2,1,2); INSERT INTO tableB2 (id,af,bf,cf) VALUES (11,12,11,11); INSERT INTO tableB2 (id,af,bf,cf) VALUES (12,11,12,12); INSERT INTO tableB2 (af,bf,cf) VALUES (13,16,13); INSERT INTO tableB2 (id,af,bf,cf) VALUES (3,3,3,3); INSERT INTO tableB2 (af,bf,cf) VALUES (14,17,14); INSERT INTO tableB2 (id,af,bf,cf) VALUES (5,15,5,5); |
檢視結果:
// 亂序 MySQL [test]> SELECT * FROM tableB2; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 1 | 1 | | 2 | 2 | 2 | 2 | | 11 | 11 | 11 | 11 | | 12 | 12 | 12 | 12 | | 13 | 13 | 13 | 13 | | 3 | 3 | 3 | 3 | | 14 | 14 | 14 | 14 | | 5 | 5 | 5 | 5 | +----+----+----+----+ 8 rows in set (0.00 sec) |
自增ID不影響查詢結果的順序,繼續看下面的操作:
MySQL [test]> SELECT COUNT(1) FROM tableB2; +----------+ | COUNT(1) | +----------+ | 8 | +----------+ 1 row in set (0.01 sec)
MySQL [test]> SELECT * FROM tableB2 WHERE id>0 LIMIT 3; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 1 | 1 | | 2 | 2 | 2 | 2 | | 3 | 3 | 3 | 3 | +----+----+----+----+ 3 rows in set (0.00 sec)
// 未亂序 MySQL [test]> SELECT * FROM tableB2 WHERE id>0 LIMIT 7; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 1 | 1 | | 2 | 2 | 2 | 2 | | 3 | 3 | 3 | 3 | | 5 | 5 | 5 | 5 | | 11 | 11 | 11 | 11 | | 12 | 12 | 12 | 12 | | 13 | 13 | 13 | 13 | +----+----+----+----+ 7 rows in set (0.00 sec)
// 亂序 MySQL [test]> SELECT * FROM tableB2 WHERE id>0 LIMIT 8; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 1 | 1 | 1 | 1 | | 2 | 2 | 2 | 2 | | 11 | 11 | 11 | 11 | | 12 | 12 | 12 | 12 | | 13 | 13 | 13 | 13 | | 3 | 3 | 3 | 3 | | 14 | 14 | 14 | 14 | | 5 | 5 | 5 | 5 | +----+----+----+----+ 8 rows in set (0.00 sec)
MySQL [jay_data]> SELECT * FROM tableB2 WHERE id>6 LIMIT 3; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 11 | 11 | 11 | 11 | | 12 | 12 | 12 | 12 | | 13 | 13 | 13 | 13 | +----+----+----+----+ 3 rows in set (0.01 sec)
MySQL [jay_data]> SELECT * FROM tableB2 WHERE id>8 LIMIT 3; +----+----+----+----+ | id | af | bf | cf | +----+----+----+----+ | 11 | 11 | 11 | 11 | | 12 | 12 | 12 | 12 | | 13 | 13 | 13 | 13 | +----+----+----+----+ 3 rows in set (0.00 sec) |
現象和自增ID為主鍵時完全相同。
4.3. 原因分析
MyISAM的索引也是B+樹結構,但索引檔案和資料檔案分開儲存在不同檔案。如果LIMIT的值達到或超過表的總記錄數,則查詢直接掃描資料檔案,因此如果不指定“ORDER BY f_id”,則返回結果和插入順序一致。但如果LIMIT的值小於表的總記錄數,則和InnoDB一樣掃描索引,因此可以不指定“ORDER BY f_id”。MyISAM的主鍵(主索引)和普通索引沒有本質區別,只是主鍵有唯一性約束,而普通索引可重複。
5. 研究結論
實際情況會更復雜,比如有修改有刪除,這些都需要是一步測試,甚至可能和版本相關。即使是聚集索引,不指定“ORDER BY f_id”,也沒法保證順序。如果對資料沒有嚴格的要求,可以考慮不指定“ORDER BY f_id”,但如果必須不多不少,則必須帶上“ORDER BY f_id”,不管是InnoDB還是MyISAM,也不管自增ID是主鍵還是非主鍵。但是對於一張大表,加上“ORDER BY f_id”後的查詢效能可能降低一個甚至更多數量級。謹記:MySQL沒有預設順序這個概念。
在使用“ORDER BY f_id”時,請指定f_id的上下限,這樣能夠保證較好的效能,比如:“WHERE f_id>=N AND f_id<=M”,否則如果只有上限或下限,效能可能會受到很大影響,建議用EXPLAIN瞭解詳情。
如果實在不想用ORDER BY f_id”,還可以考慮如下方式(query可能返回空可能是掃描完了,也可能是該段是空隙無資料):
const int step = 1000; while (true) { const std::string& sql = format_string( "SELECT f_id,f_a,f_b,f_c,f_d FROM table " "WHERE f_id BETWEEN %u AND (%u+step)", id, id); mysql.query(sql); id += step + 1; } |
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