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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_r374438429
 
 

 ##########
 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
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.
 
 Review comment:
   This is a bit to vague to be useful. First, describe how the Metastore describes a table.
Maybe, "A table is assumed to be made op of one or more files which reside in one or more
directories. Every table described by the Metastore must reside in a top-level folder; the
Metastore cannot describe bare files. (True?)
   
   If a table consists of a single directory, then it is non-partitioned. The single directory
can contain any number of files. Larger tables tend to have subdirectories. Each subdirectory
is a partition and such table are called "partitioned."
   
   It looks like the material is trying to then say how the information is stored. I would
describe that separately since there is nothing a user can do with the information.
   
   Instead, it would be better to show an example. We do hae a way to query metastore data?
Then say:
   
   "The following shows metadata for a non-partitioned table:
   
   ```
   SELECT ...
   <show output>
   ```
   
   Partitioned table use *partition keys*: the directory name describes data within that directory.
For example, the "2019" folder holds data for the year 2019. The Metastore contains an entry
for each partition. (Show an example.)
   
   Partitions can be nested: we might have subdirectories 01-12 within the 2019 folder. (Explain
how this appears when queried.)
   
   The Metastore assumes your directory names follow a convention. You may be familiar with
the Hive 'col=value` format. Drill instead uses (describe what it is.)"

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