Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

[root@mongodb-test-td-2-c ~]# echo "alter table test1 add column idd int not null AUTO_INCREMENT;alter table test1 add index idx_idd(idd);alter table test2 add column idd int not null AUTO_INCREMENT;" |soar -report-type rewrite -rewrite-rules mergealter
ALTER TABLE test1 add column idd int not null AUTO_INCREMENT, add index idx_idd(idd) ;
ALTER TABLE test2 add column idd int not null AUTO_INCREMENT ;
[root@mongodb-test-td-2-c ~]#
[root@mongodb-test-td-2-c ~]#
[root@mongodb-test-td-2-c ~]#
[root@mongodb-test-td-2-c ~]#
[root@mongodb-test-td-2-c ~]# echo "alter table test1 add column idd int not null AUTO_INCREMENT;alter table test1 alter column idd drop default;alter table test1 add index idx_idd(idd);alter table test2 add column idd int not null AUTO_INCREMENT;" |soar -report-type rewrite -rewrite-rules mergealter
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x992a10]

goroutine 1 [running]:
github.com/XiaoMi/soar/advisor.(*Query4Audit).RuleColumnWithCharset(0xc000a11e00, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
src/github.com/XiaoMi/soar/advisor/heuristic.go:2857 +0x290
main.main()
src/github.com/XiaoMi/soar/cmd/soar/soar.go:210 +0xb76
[root@mongodb-test-td-2-c ~]#

2. What did you expect to see?
   最好有一个条件判断，可以自由选择是否将drop default操作合并到alter语句中
   例如 添加一个控制参数 --ensure-drop-default=[true |false]
   参数为true时表示：祛除字段的default值
   参数为false时表示：保留字段的default值，忽略drop default语句；
   原因:有些架构平台在根据model生成sql时会产生alter table table_name alter column column_name drop default;这样的语句。一般情况下 这些默认值是要保留的，所以预留一个开关，决定是否保留drop default操作。

3. What did you see instead?
   [root@mongodb-test-td-2-c ~]# echo "alter table test1 add column idd int not null AUTO_INCREMENT;alter table test1 alter column idd drop default;alter table test1 add index idx_idd(idd);alter table test2 add column idd int not null AUTO_INCREMENT;" |soar -report-type rewrite -rewrite-rules mergealter
   panic: runtime error: invalid memory address or nil pointer dereference
   [signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x992a10]

goroutine 1 [running]:
github.com/XiaoMi/soar/advisor.(*Query4Audit).RuleColumnWithCharset(0xc000a11e00, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
src/github.com/XiaoMi/soar/advisor/heuristic.go:2857 +0x290
main.main()
src/github.com/XiaoMi/soar/cmd/soar/soar.go:210 +0xb76

4. What version of are you using (soar -version)?
   version:0.10.0-10-g1bdc431