paul-rogers commented on a change in pull request #1953: Add docs for Drill
Metastore
URL: https://github.com/apache/drill/pull/1953#discussion_r374438898
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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.
Review comment:
Again, turn this around in terms of what the user does.
"A traditional database divides tables into schemas and tables. Drill can
connect to any number of data sources, each of which may have its own schema.
As a result, the Metastore labels tables with a combination of (plugin
configuration name, workspace name, table name). Note that if you rename any of
these items, you must delete and recreate the table's Metadata entry. (We can
do that, can't we?)"
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