assertion failed on order by items

[zz-jason]

Bug Report

1. What did you do?

reproduce step:

select 1 as a from dual order by a limit 1;

2. What did you expect to see?

result in mysql:

MySQL(root@localhost:test) > select 1 as a from dual order by a limit 1;
+---+
| a |
+---+
| 1 |
+---+
1 row in set (0.00 sec)

3. What did you see instead?

result in tidb:

TiDB(root@127.0.0.1:test) > select 1 as a from dual order by a limit 1;
ERROR 2013 (HY000): Lost connection to MySQL server during query

the panic stack is:

2019/02/17 10:05:07.983 conn.go:536: [error] lastCmd select 1 as a from dual order by a limit 1, interface conversion: expression.Expression is *expression.Constant, not *expression.Column, goroutine 3560 [running]:
github.com/pingcap/tidb/server.(*clientConn).Run.func1(0xc0003a0750)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/server/conn.go:534 +0x10a
panic(0x4ff6cc0, 0xc0005c3920)
        /Users/jianzhang.zj/opt/goroot/src/runtime/panic.go:513 +0x1b9
github.com/pingcap/tidb/server.(*clientConn).writeResultset.func1(0x0, 0x53d0020, 0xc007a05c20, 0xc000707bd0, 0xc0003a0750)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/server/conn.go:1086 +0x32f
panic(0x4ff6cc0, 0xc0005c3920)
        /Users/jianzhang.zj/opt/goroot/src/runtime/panic.go:513 +0x1b9
github.com/pingcap/tidb/executor.(*SortExec).buildKeyColumns(0xc00012a8c0)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/executor/sort.go:149 +0x1c0
github.com/pingcap/tidb/executor.(*TopNExec).loadChunksUntilTotalLimit(0xc00012a8c0, 0x53c5540, 0xc000847d40, 0xc0006aa050, 0x1)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/executor/sort.go:294 +0x322
github.com/pingcap/tidb/executor.(*TopNExec).Next(0xc00012a8c0, 0x53c5540, 0xc000847d40, 0xc00000e6f8, 0x0, 0x0)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/executor/sort.go:255 +0x1e7
github.com/pingcap/tidb/executor.(*ProjectionExec).unParallelExecute(0xc0076342d0, 0x53c5540, 0xc000847d40, 0xc0005c38c0, 0x1, 0x1)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/executor/projection.go:169 +0x9a
github.com/pingcap/tidb/executor.(*ProjectionExec).Next(0xc0076342d0, 0x53c5540, 0xc000847d40, 0xc00000e6f0, 0x0, 0x0)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/executor/projection.go:158 +0xe0
github.com/pingcap/tidb/executor.(*recordSet).Next(0xc007a05bd0, 0x53c5540, 0xc000847d40, 0xc00000e6f0, 0x0, 0x0)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/executor/adapter.go:104 +0xac
github.com/pingcap/tidb/server.(*tidbResultSet).Next(0xc007a05c20, 0x53c5540, 0xc000847d40, 0xc00000e6f0, 0x4, 0x400)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/server/driver_tidb.go:365 +0x51
github.com/pingcap/tidb/server.(*clientConn).writeChunks(0xc0003a0750, 0x53c5540, 0xc000847d40, 0x53d0020, 0xc007a05c20, 0xc000007b00, 0xc0003a0750, 0xc0006aa000)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/server/conn.go:1135 +0x343
github.com/pingcap/tidb/server.(*clientConn).writeResultset(0xc0003a0750, 0x53c5540, 0xc000847d40, 0x53d0020, 0xc007a05c20, 0xc00000a000, 0x0, 0x0, 0x0)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/server/conn.go:1099 +0x1bc
github.com/pingcap/tidb/server.(*clientConn).handleQuery(0xc0003a0750, 0x53c5540, 0xc000847d40, 0xc007b0a1b1, 0x2a, 0x0, 0x0)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/server/conn.go:1016 +0x124
github.com/pingcap/tidb/server.(*clientConn).dispatch(0xc0003a0750, 0xc007b0a1b1, 0x2b, 0x2b, 0x0, 0x0)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/server/conn.go:745 +0x59a
github.com/pingcap/tidb/server.(*clientConn).Run(0xc0003a0750)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/server/conn.go:582 +0x205
github.com/pingcap/tidb/server.(*Server).onConn(0xc000679730, 0x53d27e0, 0xc0005bc200)
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/server/server.go:405 +0x224
created by github.com/pingcap/tidb/server.(*Server).Run
        /Users/jianzhang.zj/opt/gopath/src/github.com/pingcap/tidb/server/server.go:331 +0x595

the execution plan is:

TiDB(root@127.0.0.1:test) > desc select 1 as a from dual order by a limit 1;
+----------------------+-------+------+--------------------------+
| id                   | count | task | operator info            |
+----------------------+-------+------+--------------------------+
| Projection_6         | 1.00  | root | 1                        |
| └─TopN_7             | 1.00  | root | 1:asc, offset:0, count:1 |
|   └─TableDual_10     | 1.00  | root | rows:1                   |
+----------------------+-------+------+--------------------------+
3 rows in set (0.00 sec)

as you can see, the order by item in TopN_7 is a constant 1.

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

TiDB(root@127.0.0.1:test) > select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v3.0.0-beta-64-g6136ab14c
Git Commit Hash: 6136ab14cffcb21e71ffc59bd8df8de1e4af2a69
Git Branch: master
UTC Build Time: 2019-02-17 02:00:47
GoVersion: go version go1.11.2 darwin/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false
1 row in set (0.01 sec)