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Subject [GitHub] [drill] paul-rogers commented on a change in pull request #1953: Add docs for Drill Metastore
Date Tue, 04 Feb 2020 03:12:29 GMT
paul-rogers commented on a change in pull request #1953: Add docs for Drill Metastore
URL: https://github.com/apache/drill/pull/1953#discussion_r374440213
 
 

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 File path: _docs/performance-tuning/drill-metastore/010-using-drill-metastore.md
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+---
+title: "Using Drill Metastore"
+parent: "Drill Metastore"
+date: 2020-01-31
+---
+
+Drill 1.17 introduces the Drill Metastore which stores the table schema and table statistics.
Statistics allow Drill to better create optimal query plans.
+
+The Metastore is a Beta feature; it is subject to change. We encourage you to try it and
provide feedback.
+Because the Metastore is in Beta, the SQL commands and Metastore formats may change in the
next release.
+{% include startnote.html %}In Drill 1.17, this feature is supported for Parquet tables only
and is disabled by default.{% include endnote.html %}
+
+## Enabling Drill Metastore
+
+To use the Drill Metastore, you must enable it at the session or system level with one of
the following commands:
+
+	SET `metastore.enabled` = true;
+	ALTER SYSTEM SET `metastore.enabled` = true;
+
+Alternatively, you can enable the option in the Drill Web UI at `http://<drill-hostname-or-ip-address>:8047/options`.
+
+## Computing and storing table metadata to Drill Metastore
+
+Once you enable the Metastore, the next step is to populate it with data. Drill can query
a table whether that table
+ has a Metastore entry or not. (If you are familiar with Hive, then you know that Hive requires
that all tables have
+ Hive Metastore entries before you can query them.) In Drill, only add data to the Metastore
when doing so improves
+ query performance. In general, large tables benefit from statistics more than small tables
do.
+
+Unlike Hive, Drill does not require you to declare a schema. Instead, Drill infers the schema
by scanning your table 
+ in the same way as it is done during regular select and computes some metadata like `MIN`
/ `MAX` column values and
+ `NULLS_COUNT` designated as "metadata" to be able to produce more optimizations like filter
push-down, etc. If
+ `planner.statistics.use` option is enabled, this command will also calculate and store table
statistics into Drill
+ Metastore.
+
+## Configuration
+
+Default Metastore configuration is defined in `drill-metastore-default.conf` file.
+It can be overridden in `drill-metastore-override.conf`. Distribution configuration can be
+indicated in `drill-metastore-distrib.conf`.
+
+All configuration properties should reside in `drill.metastore` namespace.
+Metastore implementation based on class implementation config property `drill.metastore.implementation.class`.
+The default value is the following:
+
+```
+drill.metastore: {
+  implementation.class: "org.apache.drill.metastore.iceberg.IcebergMetastore"
+}
+```
+
+Note, that currently out of box Iceberg Metastore is available and is the default one. Though
any custom
+ implementation can be added by placing the JAR into classpath which has the implementation
of
+ `org.apache.drill.metastore.Metastore` interface and indicating custom class in the `drill.metastore.implementation.class`.
+
+### Metastore Components
+
+Metastore can store metadata for various components: tables, views, etc.
+Current implementation provides fully functioning support for tables component.
+Views component support is not implemented but contains stub methods to show
+how new Metastore components like UDFs, storage plugins, etc. can be added in the future.
+
+### Metastore Tables
+
+Metastore Tables component contains metadata about Drill tables, including general information,
as well as
+information about table segments, files, row groups, partitions.
+
+Full table metadata consists of two major concepts: general information and top-level segments
metadata.
+Table general information contains basic table information and corresponds to the `BaseTableMetadata`
class.
+
+A table can be non-partitioned and partitioned. Non-partitioned tables have only one top-level
segment 
+which is called default (`MetadataInfo#DEFAULT_SEGMENT_KEY`). Partitioned tables may have
several top-level segments.
+Each top-level segment can include metadata about inner segments, files, row groups, and
partitions.
+
+A unique table identifier in Metastore Tables is a combination of storage plugin, workspace,
and table name.
+Table metadata inside is grouped by top-level segments, unique identifier of the top-level
segment and its metadata
+is storage plugin, workspace, table name, and metadata key.
+
+### Related Session/System Options
+
+The following options are set via `ALTER SYSTEM SET`, or `ALTER SESSION SET` or via the Drill
Web console.
+
+- **metastore.enabled**
+Enables Drill Metastore usage to be able to store table metadata during ANALYZE TABLE commands
execution and to be able
+ to read table metadata during regular queries execution or when querying some INFORMATION_SCHEMA
tables. Default is `false`.
+- **metastore.metadata.store.depth_level**
+Specifies the maximum level of depth for collecting metadata. Same options as the _level_
option above. Default is `'ALL'`.
+- **metastore.retrieval.retry_attempts**
+If you run the `ANALYZE TABLE` command at the same time as queries run, then the query can
read incorrect or corrupt statistics.
+Drill will reload statistics and replan the query. This option specifies the maximum number
of retry attempts. Default is `5`.
+- **metastore.metadata.fallback_to_file_metadata**
+Allows using [file metadata cache]({{site.baseurl}}/docs/refresh-table-metadata) for the
case when required metadata is absent in the Metastore.
+Default is `true`.
+- **metastore.metadata.use_schema**
+The `ANALYZE TABLE` command infers table schema as it gathers statistics. This option tells
Drill to use that schema information while planning the query. 
+Disable this option if Drill has inferred the schema incorrectly, or schema will be provided
separately (see [CREATE OR REPLACE SCHEMA]({{site.baseurl}}/docs/create-or-replace-schema)).
+Default is `true`.
+- **metastore.metadata.use_statistics**
+Enables obtaining table and column statistics, stored in the Metastore, at the planning stage.
Default is `true`.
+Enable `planner.statistics.use` to be able to use statistics during query planning.
+- **metastore.metadata.ctas.auto-collect**
+Drill provides the [`CREATE TABLE AS`]({{site.baseurl}}/docs/create-or-replace-schema) commands
to create new tables.
+This option causes Drill to gather schema and statistics for those tables automatically as
they are written.
+This option is not implemented for now. Possible values: `'ALL'`, `'SCHEMA'`, `'NONE'`. Default
is `'NONE'`.
 
 Review comment:
   If not implemented, don't document it. Save the documentation for when it is implemented.
(That is, we don't document future features.)

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