索引分裂的enq
索引分裂引起的enq-index contention和enq: TX - allocate ITL entry，该等待事件是由于索引分裂下，并发的用户在此更新索引导致。
SQL> create table t1(id number,name varchar2(1000));
Table created
SQL> create index index_t1 on t1(id,name) pctfree 0;
Index created
SQL> begin
2 for i in 1..1000 loop
3 insert into t1 values(i,rpad('x',1000,'x'));
4 end loop;
5 commit;
6 end;
7 /
PL/SQL procedure successfully completed
SQL> analyze index index_t1 validate structure;
Index analyzed
SQL> select name,height,blocks,lf_rows,lf_blks,pct_used from index_stats;
NAME HEIGHT BLOCKS LF_ROWS LF_BLKS PCT_USED
------------------------------ ---------- ---------- ---------- ---------- ----------
INDEX_T1 2 256 1000 143 89
SQL>
SQL> select a.value,b.name from v$mystat a,v$statname b
2 where a.statistic#=b.statistic# and b.name like 'leaf node%'
3 ;
VALUE NAME
---------- ----------------------------------------------------------------
156 leaf node splits
156 leaf node 90-10 splits
SQL> insert into t1 values(1,rpad('x',1000,'x'));
1 row inserted
SQL> commit;
SQL> analyze index index_t1 validate structure;
Index analyzed
SQL> select name,height,blocks,lf_rows,lf_blks,pct_used from index_stats;
NAME HEIGHT BLOCKS LF_ROWS LF_BLKS PCT_USED
------------------------------ ---------- ---------- ---------- ---------- ----------
INDEX_T1 2 256 1001 144 88
SQL>
SQL> select a.value,b.name from v$mystat a,v$statname b
2 where a.statistic#=b.statistic# and b.name like 'leaf node%'
3 ;
VALUE NAME
---------- ----------------------------------------------------------------
157 leaf node splits
156 leaf node 90-10 splits
发生索引叶块的5-5分裂，叶块由143变为了144.
SQL> insert into t1 values(0.5,rpad('x',1000,'x'));
1 row inserted
SQL> analyze index index_t1 validate structure;
Index analyzed
SQL> select name,height,blocks,lf_rows,lf_blks,pct_used from index_stats;
NAME HEIGHT BLOCKS LF_ROWS LF_BLKS PCT_USED
------------------------------ ---------- ---------- ---------- ---------- ----------
INDEX_T1 2 256 1002 144 88
SQL> insert into t1 values(1001,rpad('x',1000,'x'));
1 row inserted
SQL> commit;
Commit complete
SQL> analyze index index_t1 validate structure;
Index analyzed
SQL> select name,height,blocks,lf_rows,lf_blks,pct_used from index_stats;
NAME HEIGHT BLOCKS LF_ROWS LF_BLKS PCT_USED
------------------------------ ---------- ---------- ---------- ---------- ----------
INDEX_T1 2 256 1003 144 89
此时没有发生叶块分裂，因为索引顺序排序下，叶块的最后的一个叶块节点并并没有写满，所以1001这个键值会存储在最后的一个叶块上。
SQL> begin
2 for i in 1002..1200 loop
3 insert into t1 values(i,rpad('x',1000,'x'));
4 end loop;
5 commit;
6 end;
7 /
PL/SQL procedure successfully completed
SQL>
SQL> select a.value,b.name from v$mystat a,v$statname b
2 where a.statistic#=b.statistic# and b.name like 'leaf node%'
3 ;
VALUE NAME
---------- ----------------------------------------------------------------
186 leaf node splits
185 leaf node 90-10 splits
对比一下上述的统计信息，重新插入数值由于在维护索引，分别发生了5-5分裂和1-9分裂
SQL> analyze index index_t1 validate structure;
Index analyzed
SQL> select name,height,blocks,lf_rows,lf_blks,pct_used from index_stats;
NAME HEIGHT BLOCKS LF_ROWS LF_BLKS PCT_USED
------------------------------ ---------- ---------- ---------- ---------- ----------
INDEX_T1 2 256 1202 173 88
模拟下关于索引等待的两个事件
并发的两个session执行下列pl/sql
SQL> declare
2 begin
3 for i in 501..1000 loop
4 insert into t1 values(i,rpad('x',1000,'x'));
5 end loop;
6 commit;
7 end;
8 /
PL/SQL procedure successfully completed
SQL> select userenv('SID') from dual;
USERENV('SID')
--------------
144
SQL> select sid,event,total_waits,time_waited,average_wait from v$session_event where event like 'enq%';
SID EVENT TOTAL_WAITS TIME_WAITED AVERAGE_WAIT
---------- ---------------------------------------------------------------- ----------- ----------- ------------
138 enq: HW - contention 428 664 1.55
138 enq: TX - index contention 7 2 0.28
144 enq: RO - fast object reuse 4 54 13.62
144 enq: HW - contention 592 1448 2.45
144 enq: TX - allocate ITL entry 4 0 0.01
144 enq: TX - index contention 2 21 10.49
6 rows selected
等待出现了，enq: TX - allocate ITL entry和enq: TX - index contention
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