Repairing Hive or Impala partitions

Under certain conditions, you must manually repair metadata about Hive or Impala partitions. You learn when and how to do this.

Partition information in the metadata gets stale under the following circumstances:

Hive: The dynamic partition refresh is disabled.
Impala: Data files are removed by a non-Impala mechanism, and the table metadata is not updated

You must manually refresh metastore partition information when these conditions exist.

This task assumes you created a partitioned external table named students_part that stores partitions outside the warehouse. You remove one of the partition directories on the object store. This action renders the metastore inconsistent with S3. You repair the discrepancy manually to synchronize the metastore with S3 as follows;

Hive
To repair partition metadata, you run the MSCK (metastore consistency check) Hive command to manually add partitions that are added to or removed from the object store, but are not present in the Hive metastore.

Impala
To repair partition metadata, you run ALTER TABLE with the RECOVER PARTITIONS clause to to find any new partition directories and the data files.

Remove a partition directory, called engineering for example, from S3.
In Hive or Impala, look at the students_part table partitions.
```
SHOW PARTITIONS students_part;
```
The list of partitions is stale in the metadata because it still includes the major=engineering partition, so you must run MSCK to repair the metadata.
```
+-------------------+
|       major       |
+-------------------+
| major=engineering |
| major=humanities  |
| major=mathematics |
+-------------------+
```

Repair the partition.

Hive example:

MSCK REPAIR TABLE students_part DROP PARTITIONS;

Impala example:

ALTER TABLE students_part RECOVER PARTITIONS;

Show that the repair succeeded.

SHOW PARTITIONS students_part;

The repair was successful. The sales partition no longer appears.

+-------------------+
|       major       |
+-------------------+
| major=humanities  |
| major=mathematics |
+-------------------+