Index merge doesn't take effect when using prefix key

[lilinghai]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

create table t(a int,b char(100),key(a),key(b(10));
desc select /*+ use_index_merge(t) */ * from t where a=10 or b="x";

2. What did you expect to see?

use_index_merge takes effect

3. What did you see instead?

+-------------------------+----------+-----------+---------------+-------------------------------------+
| id                      | estRows  | task      | access object | operator info                       |
+-------------------------+----------+-----------+---------------+-------------------------------------+
| TableReader_7           | 8000.00  | root      |               | data:Selection_6                    |
| └─Selection_6           | 8000.00  | cop[tikv] |               | or(eq(d1.t.a, 10), eq(d1.t.b, "x")) |
|   └─TableFullScan_5     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo      |
+-------------------------+----------+-----------+---------------+-------------------------------------+
3 rows in set, 1 warning (0.01 sec)

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

Release Version: v4.0.0-rc-33-gd69868961
Git Commit Hash: d698689
Git Branch: release-4.0
UTC Build Time: 2020-04-15 02:59:00
GoVersion: go1.13.8
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[zz-jason]

@lzmhhh123 PTAL

[qw4990]

@tangwz Do you willing to help us fix this?

[tangwz]

@qw4990 I will give a try.

[tangwz]

/assign

[tangwz]

I saw a comment in function accessPathsForConds(conditions []expression.Expression, usedIndexCount int):

// If AccessConds is empty or tableFilter is not empty, we ignore the access path.
// Now these conditions are too strict.

I think for prefix key we need change these conditions especially the condition tableFilter is not empty.

Should I change that limit?