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create table t1(c1 integer not null,c2 integer not null, key (c1)) ENGINE=InnoDB STATS_PERSISTENT=1; create view v1 as select * from t1 where c1 in (0,1); insert t1 select 0,seq from seq_1_to_500; insert t1 select 1,seq from seq_1_to_100; insert t1 select 2,seq from seq_1_to_50; insert t1 select 3,seq from seq_1_to_20; analyze table t1; Table Op Msg_type Msg_text test.t1 analyze status Engine-independent statistics collected test.t1 analyze status OK # # Delete with limit (quick select - range acces) # start transaction; delete from t1 where (select count(*) from t1 b where b.c1=t1.c1) = 500 limit 1; affected rows: 1 delete from t1 where (select count(*) from t1 b where b.c1=t1.c1) = 500 limit 1; affected rows: 0 select count(*) from v1 where c1=0; count(*) 499 rollback; # # Delete # start transaction; delete from t1 where (select count(*) from t1 b where b.c1=t1.c1) = 500 ; affected rows: 500 rollback; # # Delete with exists # start transaction; select count(*) from v1 where c1=2; count(*) 0 delete from t1 where c1=2 and exists(select 'x' from t1 b where b.c2<10); affected rows: 50 delete from t1 where c1=2 and exists(select 'x' from t1 b where b.c2<10); affected rows: 0 select count(*) from v1 where c1=2; count(*) 0 rollback; # # Delete through a view with limit (range access) # start transaction; explain delete from v1 where (select count(*) from t1 b where b.c1=v1.c1) = 500 limit 1; id select_type table type possible_keys key key_len ref rows Extra 1 PRIMARY t1 range c1 c1 4 NULL 600 Using where 2 DEPENDENT SUBQUERY b ref c1 c1 4 test.t1.c1 167 Using index delete from v1 where (select count(*) from t1 b where b.c1=v1.c1) = 500 limit 1; affected rows: 1 delete from v1 where (select count(*) from t1 b where b.c1=v1.c1) = 500 limit 1; affected rows: 0 select count(*) from v1 where c1=0; count(*) 499 rollback; # # Delete through a view (ALL access) # start transaction; explain delete from v1 where (select count(*) from t1 b where b.c1=v1.c1) = 500; id select_type table type possible_keys key key_len ref rows Extra 1 PRIMARY t1 ALL c1 NULL NULL NULL # Using where 2 DEPENDENT SUBQUERY b ref c1 c1 4 test.t1.c1 # Using index delete from v1 where (select count(*) from t1 b where b.c1=v1.c1) = 500 ; affected rows: 500 select count(*) from v1 where c1=0; count(*) 0 rollback; # # Delete failed due to trigger # create trigger trg after delete on t1 for each row begin declare c int; begin if old.c1 = 1 then select count(*) into c from t1 where c1!=old.c1; SIGNAL SQLSTATE '45000' set table_name=c; end if; end; end; / start transaction; delete from t1 where c1=1 and (select count(*) from t1 b where b.c1=t1.c1) > 0 order by c2 asc limit 10; ERROR 45000: Unhandled user-defined exception condition rollback; start transaction; delete from t1 where (select count(*) from t1 b where b.c1=t1.c1) > 0 order by c1 desc limit 100; ERROR 45000: Unhandled user-defined exception condition select c1,count(*) from t1 group by c1; c1 count(*) 0 500 1 100 2 50 3 20 rollback; drop trigger trg; # # Delete through a view with returning # start transaction; delete from t1 where (select count(*) from t1 b where b.c1=t1.c1) = 500 order by c2 asc limit 10 returning c1,c2; c1 c2 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 10 rollback; start transaction; delete from t1 where (select count(*) from t1 b where b.c1=t1.c1) = 500 order by c2 desc limit 10 returning c1,c2; c1 c2 0 491 0 492 0 493 0 494 0 495 0 496 0 497 0 498 0 499 0 500 rollback; drop view v1; drop table t1; # # Delete from table with more than 150000 rows # create table t1(c1 integer not null,c2 integer not null, key (c1)); insert t1 select 0,seq from seq_1_to_128000; insert t1 select 1,seq from seq_1_to_25600; select count(*) from t1; count(*) 153600 # with a lot of memory for sort_buffer_size set session sort_buffer_size = 1024000; delete from t1 where c1=0 and exists(select 'x' from t1 b where b.c1<10); affected rows: 128000 # with little memory for sort_buffer_size insert t1 select 0,seq from seq_1_to_128000; set session sort_buffer_size = 1024; delete from t1 where c1=0 and exists(select 'x' from t1 b where b.c1<10); affected rows: 128000 drop table t1;