KingbaseES Truncate 與 Delete 機制比較
阿新 • • 發佈:2021-08-02
使用過Oracle的都知道,Truncate操作由於不需要寫redo日誌,因此,在效能上會比delete操作更高效,但在實際使用過程中,有時會發現delete比truncate速度更快。以下介紹下二者的機制,讓大家對二者有清晰的瞭解。
一、大資料量刪除操作
1、資料準備
建立兩張沒有索引的大表:t1 , t2 ,每張表的資料量接近 350W
test=# create table t2 as select * from t1; SELECT 3461120
2、測試Delete操作
test=# checkpoint; CHECKPOINT test=# select pg_current_wal_lsn(); pg_current_wal_lsn-------------------- 0/92DB1FC8 (1 row) test=# select relname,relfilenode from sys_class where relname='t1'; relname | relfilenode ---------+------------- t1 | 16575 (1 row) test=# delete from t1; DELETE 3461120 Time: 4771.554 ms (00:04.772) test=# select pg_current_wal_lsn(); pg_current_wal_lsn-------------------- 0/C87A2EC0 (1 row) test=# select pg_wal_lsn_diff('0/C87A2EC0','0/92DB1FC8'); pg_wal_lsn_diff ----------------- 899616504 (1 row)
test=# checkpoint; CHECKPOINT
test=# select relname,relfilenode from sys_class where relname='t1'; relname | relfilenode ---------+------------- t1 |16575 (1 row)
結論:Delete 操作產生了近900M的日誌;資料檔案的relfilenode 不變。
3、測試Truncate操作
test=# checkpoint; CHECKPOINT test=# select relname,relfilenode from sys_class where relname='t2'; relname | relfilenode ---------+------------- t2 | 16581 (1 row) test=# select pg_current_wal_lsn(); pg_current_wal_lsn -------------------- 0/CAA7C678 (1 row) test=# truncate table t2; TRUNCATE TABLE Time: 84.124 ms test=# select pg_current_wal_lsn(); pg_current_wal_lsn -------------------- 0/CAA82D40 (1 row) Time: 0.327 ms test=# select relname,relfilenode from sys_class where relname='t2'; relname | relfilenode ---------+------------- t2 | 16587 (1 row) Time: 0.491 ms test=# select pg_wal_lsn_diff('0/CAA82D40','0/CAA7C678'); pg_wal_lsn_diff ----------------- 26312 (1 row) Time: 1.239 ms
結論:Truncate 操作基本不產生redo;relfilenode 會變化,這是由於truncate操作相當於新建了個檔案。
三、小資料量刪除操作比較
1、比較一:資料未寫回磁碟前,進行truncate
test=# declare test-# v_sql text; test-# begin test-# for i in 1..1000 loop test-# drop table if exists t1; test-# create table t1 as select * from pg_class; test-# delete from t1; test-# end loop; test-# end; test-# / ANONYMOUS BLOCK Time: 7173.891 ms (00:07.174) test=# declare test-# v_sql text; test-# begin test-# for i in 1..1000 loop test-# drop table if exists t1; test-# create table t1 as select * from pg_class; test-# truncate t1; test-# end loop; test-# end; test-# / ANONYMOUS BLOCK Time: 7477.366 ms (00:07.477)
結論:truncate 似乎更慢。從作業系統IO看,二者的IO 相差不大。
可能原因:truncate 在作業系統層面的操作實際刪除舊檔案,新建新檔案。當呼叫作業系統命令刪除舊檔案時,需要將資料先寫回檔案,才能刪除。而本例中,實際從create table 到truncate 時間很短,資料還未寫回檔案。
2、測試二:先建表,在刪除資料
declare v_sql text; begin for i in 1..1000 loop v_sql = 'drop table if exists t'||i; execute immediate v_sql; v_sql = 'create table t'||i||' as select * from pg_class'; execute immediate v_sql; end loop; end; checkpoint; declare v_sql text; begin for i in 1..1000 loop v_sql = 'delete from t'||i; execute immediate v_sql; end loop; end; / ANONYMOUS BLOCK Time: 3412.780 ms (00:03.413) declare v_sql text; begin for i in 1..1000 loop v_sql = 'truncate t'||i; execute immediate v_sql; end loop; end; / ANONYMOUS BLOCK Time: 1268.753 ms (00:01.269)
結論:先建表,再checkpoint ,將資料寫回資料檔案。然後,再比較Delete 和 Truncate 操作,可以看到即使小表,truncate 操作也快很多。
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