planner: PartitionUnion is not supported by tidb_decode_plan

[qw4990]

Bug Report

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1. Minimal reproduce step (Required)

mysql> select tidb_decode_plan('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'); 
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| tidb_decode_plan('ngrwXjAJM185CTAJMjAwMDIJdGVzdC50LmEsIHNsZWVwKDQpLT5Db2x1bW4jMwkyCXRpbWU6NHMsIGxvb3BzOjMsIENvbmN1cnJlbmN5OjUJNS40NSBLQglOL0EKMQkwXzEwFVoACQ0+GDMxMS40wrUZRAwJTi9BAQSgCjIJMzFfMTIJMAkxMDAwMAlkYXRhOlRhYmxlRnVsbFNjYW5fMTEJMAkFhRAxMzEuOB1HbDEsIG                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
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|       id                      task            estRows operator info                                           actRows execution info                                                                                                                          memory   disk
        Projection_9            root            20002   test.t.a, sleep(4)->Column#3                            2       time:4s, loops:3, Concurrency:5                                                                                                         5.45 KB  N/A
        └─UnknownPlanID0_10     root            20002                                                           2       time:311.4µs, loops:3                                                                                                                   N/A      N/A
          ├─TableReader_12      root            10000   data:TableFullScan_11                                   0       time:131.8µs, loops:1, cop_task: {num: 1, max: 91.4µs, proc_keys: 0, rpc_num: 1, rpc_time: 79.3µs, copr_cache_hit_ratio: 0.00}  151 Bytes       N/A
          │ └─TableFullScan_11  cop[tikv]       10000   table:t, partition:p0, keep order:false, stats:pseudo   0       tikv_task:{time:0s, loops:0}                                                                                                            N/A      N/A
          ├─TableReader_14      root            1       data:TableFullScan_13                                   1       time:124.3µs, loops:2, cop_task: {num: 1, max: 77.3µs, proc_keys: 0, rpc_num: 1, rpc_time: 69µs, copr_cache_hit_ratio: 0.00}    158 Bytes       N/A
          │ └─TableFullScan_13  cop[tikv]       1       table:t, partition:p1, keep order:false, stats:pseudo   1       tikv_task:{time:1.37µs, loops:1}                                                                                                        N/A      N/A
          ├─TableReader_16      root            1       data:TableFullScan_15                                   1       time:118.4µs, loops:2, cop_task: {num: 1, max: 76.1µs, proc_keys: 0, rpc_num: 1, rpc_time: 66.9µs, copr_cache_hit_ratio: 0.00}  158 Bytes       N/A
          │ └─TableFullScan_15  cop[tikv]       1       table:t, partition:p2, keep order:false, stats:pseudo   1       tikv_task:{time:1.11µs, loops:1}                                                                                                        N/A      N/A
          └─TableReader_18      root            10000   data:TableFullScan_17                                   0       time:142.5µs, loops:1, cop_task: {num: 1, max: 84.9µs, proc_keys: 0, rpc_num: 1, rpc_time: 61.5µs, copr_cache_hit_ratio: 0.00}  151 Bytes       N/A
            └─TableFullScan_17  cop[tikv]       10000   table:t, partition:p3, keep order:false, stats:pseudo   0       tikv_task:{time:0s, loops:0}                                                                                                            N/A      N/A |
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2. What did you expect to see? (Required)

The UnknownPlanID0_10 should be PartitionUnion

3. What did you see instead (Required)

UnknownPlanID0_10

4. What is your TiDB version? (Required)

Master