索引争用是和索引块分裂直接相关的。当一个事务需要向一个数据块插入数据时,该数据块正好发生了索引分裂,则插入事务需要等待分裂事务(注意:分裂事务不是用户事务,而是一个递归事务)。这一等待通常在高并发的OLTP系统中可见:
HELLODBA.COM> create table tx_index_contention (a number, b varchar2(1446), c date); Table created. HELLODBA.COM> create index tx_index_contention_idx1 on tx_index_contention (c, b) tablespace idx_2k pctfree 10; Index created. --session 1,产生大量的索引块分裂: HELLODBA.COM> conn demo/demo Connected. HELLODBA.COM> begin 2 for i in 1..2000 3 loop 4 insert into tx_index_contention (a, b, c) values (i, lpad('A', 1000, 'A'), sysdate); 5 end loop; 6 end; 7 / --session 2,在索引分裂的同时,插入数据: HELLODBA.COM> conn demo/demo Connected. HELLODBA.COM> alter session set events '10046 trace name context forever, level 12'; Session altered. HELLODBA.COM> begin 2 for i in 1..1000 3 loop 4 insert into tx_index_contention (a, b, c) values (i, lpad('A', 20, 'A'), sysdate); 5 end loop; 6 end; 7 / HELLODBA.COM> alter session set events '10046 trace name context off'; Session altered. HELLODBA.COM> select sid, event, total_waits from v$session_event where sid in (select sid from v$mystat) and event = 'enq: TX - index contention'; SID EVENT TOTAL_WAITS ---------- ---------------------------- ------------------- 307 enq: TX - index contention 10 |