[HIVE-2246] Dedupe tables' column schemas from partitions in the metastore db

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.8.0
• Component/s: Metastore
• Release Note: This makes an incompatible change in the metastore DB table schema from previous versions (<0.8). Older metastores created with previous versions of Hive will need to be upgraded with the supplied scripts.
• Tags: metastore, schema, JDO
• Assignee: Sohan Jain
• Reporter: Sohan Jain
• Created: 30/Jun/11 22:07
• Updated: 19/Mar/21 17:09
• Resolved: 09/Aug/11 22:08

Description

Note: this patch proposes a schema change, and is therefore incompatible with the current metastore.

We can re-organize the JDO models to reduce space usage to keep the metastore scalable for the future. Currently, partitions are the fastest growing objects in the metastore, and the metastore keeps a separate copy of the columns list for each partition. We can normalize the metastore db by decoupling Columns from Storage Descriptors and not storing duplicate lists of the columns for each partition.

An idea is to create an additional level of indirection with a "Column Descriptor" that has a list of columns. A table has a reference to its latest Column Descriptor (note: a table may have more than one Column Descriptor in the case of schema evolution). Partitions and Indexes can reference the same Column Descriptors as their parent table.

Currently, the COLUMNS table in the metastore has roughly (number of partitions + number of tables) * (average number of columns pertable) rows. We can reduce this to (number of tables) * (average number of columns per table) rows, while incurring a small cost proportional to the number of tables to store the Column Descriptors.

Please see the latest review board for additional implementation details.

Issue Links

• is required by: HIVE-2367 Indexes' storage descriptors' columns are not deduped, and altering an index leaves behind an unused storage descriptor
• is required by: HIVE-2368 Slow dropping of partitions caused by full listing of storage descriptors
• relates to: HIVE-24870 Metastore: cleanup unused column descriptors asynchronously in batches
• relates to: HIVE-24911 Metastore: Create index on SDS.CD_ID for Postgres
• requires: HIVE-2366 Metastore upgrade scripts for HIVE-2246 do not migrate indexes nor rename the old COLUMNS table