paul-rogers commented on a change in pull request #1953: Add docs for Drill
Metastore
URL: https://github.com/apache/drill/pull/1953#discussion_r374434708
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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
Review comment:
The term "Metastore Tables" is misleading. I read it like "Hive Tables": the
relational DB tables which make up Hives internal DB schema. I think you mean,
"Table Metadata": the metadata about tables which is stored in the Metastore.
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