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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 03:12:28 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_r374436719

 File path: _docs/performance-tuning/drill-metastore/010-using-drill-metastore.md
 @@ -0,0 +1,408 @@
+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
+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
+ `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
 Review comment:
   This is user documentation; users probably won't look at source code to figure out what's
what. A better approach is to say, "The detailed metadata schema is described (here). You
can try out the metadata, and get a sense of what is available, by using the Metadata tutorial."
(We do have one, complete with a sample data set? If not, we should add one.)
   The class-level information could go into a "Metastore API Guide" targeted at developers.

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