vvysotskyi commented on a change in pull request #1953: Add docs for Drill
Metastore
URL: https://github.com/apache/drill/pull/1953#discussion_r373564880
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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: 2020-01-30
+---
+
+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 an 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
+ 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.
+
+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.
+
+## 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`.
+
+### 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. be added in
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
`BaseTableMetadata` class.
+
+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.
+
+Unique table identifier in Metastore Tables is 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 maximum level 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 active for now. Possible values: `'ALL'`, `'SCHEMA'`,
`'NONE'`. Default is `'NONE'`.
+- **drill.exec.storage.implicit.last_modified_time.column.label**
+Sets the implicit column name for the last modified time (`lmt`) column. Used
when producing Metastore analyze.
+- **drill.exec.storage.implicit.row_group_index.column.label**
+Sets the implicit column name for the row group index (`rgi`) column. Used
when producing Metastore analyze.
+- **drill.exec.storage.implicit.row_group_length.column.label**
+Sets the implicit column name for the row group length (`rgl`) column. Used
when producing Metastore analyze.
+- **drill.exec.storage.implicit.row_group_start.column.label**
+Sets the implicit column name for the row group start (`rgs`) column. Used
when producing Metastore analyze.
+
+## Incremental analysis
+
+If you have computed statistics for a table, and issue `ANALYZE TABLE` a
second time, Drill will attempt to update statistics, called "incremental
analysis."
+Incremental analysis will compute metadata only for files and partitions
changed since the last analysis and reuse
+ actual metadata from the Metastore where possible.
+
+Drill performs incremental analysis only when the `ANALYZE TABLE command` is
identical to the previous command:
+- The list of columns in the `COLUMNS` clause is a subset of interesting
columns from the previous run.
+- The metadata level in the LEVEL clause must be the same as the previous run.
+
+If either of these two conditions is false, Drill will perform a full analysis
over the entire table.
+
+## General Information
+
+- Drill 1.17 supports the Metastore and `ANALYZE TABLE` only for tables stored
as Parquet files and only when stored in the `DFS` storage plugin.
+- The first time you execute ANALYZE TABLE for a table, Drill will scan the
entire tables (all files.)
+When you next issue the same command, Drill will scan only those files added
since the previous run.
+The command will return the message if table statistics are up-to-date:
+
+
+```
+apache drill (dfs.tmp)> analyze table lineitem refresh metadata;
++-------+---------------------------------------------------------+
+| ok | summary |
++-------+---------------------------------------------------------+
+| false | Table metadata is up to date, analyze wasn't performed. |
++-------+---------------------------------------------------------+
+```
+
+### Metadata usage
+
+Drill uses the Metastore in several places. When you run a query with multiple
directories, files or Parquet row groups,
+Drill will use statistics to "prune" the scan. That is, to identify those
directories, files or row groups which
+do not contain data which your query needs. If you add new files or
directories, and do not rerun `ANALYZE TABLE`,
+then Drill will assume that existing metadata is invalid and wouldn't use it.
Periodically rerun `ANALYZE TABLE` so that
+ Drill can use table metadata when possible.
+
+### Limitations
+
+This feature is currently in the alpha phase (preview, experimental) for Drill
1.17 and only applies to Parquet
+ tables in this release. You must enable this feature through the
`metastore.enabled` system/session option.
+
+## Examples
+
+Examples throughout this topic use the files and directories described in the
following section, Directory and File Setup.
+
+### Directory and File Setup
+
+Download [TPC-H sf1
tables](https://s3-us-west-1.amazonaws.com/drill-public/tpch/sf1/tpch_sf1_parquet.tar.gz)
and unpack
+ archive.
+
+Create lineitem directory in `/tmp/` and two subdirectories under
`/tmp/lineitem` named `s1` and `s2` and copy there table data:
+
+ mkdir /tmp/lineitem
+ mkdir /tmp/lineitem/s1
+ mkdir /tmp/lineitem/s2
+ cp TPCH/lineitem /tmp/lineitem/s1
+ cp TPCH/lineitem /tmp/lineitem/s2
+
+Query the directory `/tmp/lineitem`:
+
+```
+SELECT count(*) FROM dfs.tmp.lineitem;
++----------+
+| EXPR$0 |
++----------+
+| 12002430 |
++----------+
+1 row selected (0.291 seconds)
+```
+
+Notice that the query plan contains group scan with `usedMetastore = false`:
+
+
+```
+00-00 Screen : rowType = RecordType(BIGINT EXPR$0): rowcount = 1.0,
cumulative cost = {2.1 rows, 2.1 cpu, 1.0 io, 0.0 network, 0.0 memory}, id =
8410
+00-01 Project(EXPR$0=[$0]) : rowType = RecordType(BIGINT EXPR$0):
rowcount = 1.0, cumulative cost = {2.0 rows, 2.0 cpu, 1.0 io, 0.0 network, 0.0
memory}, id = 8409
+00-02 DirectScan(groupscan=[selectionRoot = file:/tmp/lineitem,
numFiles = 12, usedMetadataSummaryFile = false, usedMetastore = false, ...
+```
+
+### Computing and storing table metadata to Drill Metastore
+
+Run [ANALYZE TABLE]({{site.baseurl}}/docs/analyze-table-refresh-metadata)
command on the table, whose metadata should
+ be computed and stored into the Drill Metastore:
+
+
+```
+apache drill> ANALYZE TABLE dfs.tmp.lineitem REFRESH METADATA;
++------+-------------------------------------------------------------+
+| ok | summary |
++------+-------------------------------------------------------------+
+| true | Collected / refreshed metadata for table [dfs.tmp.lineitem] |
++------+-------------------------------------------------------------+
+1 row selected (32.257 seconds)
+```
+
+The output of this command provides the status of the command execution and
its summary.
+
+Once, its metadata is collected and stored, it will be used when querying the
table. To ensure that it was used, its
Review comment:
Thanks, updated.
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