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new e2c7e77310 Views: Clean up and clarify the view spec (#7416)
e2c7e77310 is described below
commit e2c7e77310329b2113d4122eda4bd0d46e1d252d
Author: Ryan Blue <[email protected]>
AuthorDate: Mon Apr 24 16:21:15 2023 -0700
Views: Clean up and clarify the view spec (#7416)
---
format/view-spec.md | 323 +++++++++++++++++++++++++++++++---------------------
1 file changed, 194 insertions(+), 129 deletions(-)
diff --git a/format/view-spec.md b/format/view-spec.md
index 5490a7b256..efd46c6faf 100644
--- a/format/view-spec.md
+++ b/format/view-spec.md
@@ -36,13 +36,14 @@ Each compute engine stores the metadata of the view in its
proprietary format in
## Overview
View metadata storage mirrors how Iceberg table metadata is stored and
retrieved. View metadata is maintained in metadata files. All changes to view
state create a new view metadata file and completely replace the old metadata
using an atomic swap. Like Iceberg tables, this atomic swap is delegated to the
metastore that tracks tables and/or views by name. The view metadata file
tracks the view schema, custom properties, current and past versions, as well
as other metadata.
-Each metadata file is self-sufficient. It contains the history of the last few
operations performed on the view and can be used to roll back the view to a
previous version.
+
+Each metadata file is self-sufficient. It contains the history of the last few
versions of the view and can be used to roll back the view to a previous
version.
### Metadata Location
An atomic swap of one view metadata file for another provides the basis for
making atomic changes. Readers use the version of the view that was current
when they loaded the view metadata and are not affected by changes until they
refresh and pick up a new metadata location.
-Writers create view metadata files optimistically, assuming that the current
metadata location will not be changed before the writer’s commit. Once a writer
has created an update, it commits by swapping the view's metadata file pointer
from the base location to the new location.
+Writers create view metadata files optimistically, assuming that the current
metadata location will not be changed before the writer's commit. Once a writer
has created an update, it commits by swapping the view's metadata file pointer
from the base location to the new location.
## Specification
@@ -55,68 +56,110 @@ Writers create view metadata files optimistically,
assuming that the current met
The view version metadata file has the following fields:
-| Required/Optional | Field Name | Description |
-|-------------------|------------|-------------|
-| Required | format-version | An integer version number for the view format.
Currently, this must be 1. Implementations must throw an exception if the
view's version is higher than the supported version. |
-| Required | location | The view's base location. This is used to determine
where to store view metadata files. |
-| Required | current-version-id | Current version of the view. Set to ‘1’ when
the view is first created. |
-| Optional | properties | A string to string map of view properties. This is
used for metadata such as "comment" and for settings that affect view
maintenance. This is not intended to be used for arbitrary metadata. |
-| Required | versions | An array of structs describing the known versions of
the view. The number of versions to retain is controlled by the table property:
“version.history.num-entries”. See section [Versions](#versions). |
-| Required | version-log | A list of timestamp and version ID pairs that
encodes changes to the current version for the view. Each time the
current-version-id is changed, a new entry should be added with the
last-updated-ms and the new current-version-id. |
-| Optional | schemas | A list of schemas, the same as the ‘schemas’ field from
Iceberg table spec. |
-| Optional | current-schema-id | ID of the current schema of the view |
+| Requirement | Field name | Description |
+|-------------|----------------------|-------------|
+| _required_ | `format-version` | An integer version number for the view
format; must be 1 |
+| _required_ | `location` | The view's base location; used to
create metadata file locations |
+| _optional_ | `current-schema-id` | ID of the current schema of the view,
if known |
+| _optional_ | `schemas` | A list of known schemas |
+| _required_ | `current-version-id` | ID of the current version of the view
(`version-id`) |
+| _required_ | `versions` | A list of known [versions](#versions)
of the view [1] |
+| _required_ | `version-log` | A list of [version log](#version-log)
entries with the timestamp and `version-id` for every change to
`current-version-id` |
+| _optional_ | `properties` | A string to string map of view
properties [2] |
+
+Notes:
+1. The number of versions to retain is controlled by the table property:
`version.history.num-entries`.
+2. Properties are used for metadata such as `comment` and for settings that
affect view maintenance. This is not intended to be used for arbitrary metadata.
#### Versions
-Field "versions" is an array of structs with the following fields:
+Each version in `versions` is a struct with the following fields:
-| Required/Optional | Field Name | Description |
-|-------------------|------------|-------------|
-| Required | version-id | Monotonically increasing id indicating the version
of the view. Starts with 1. |
-| Required | timestamp-ms | Timestamp expressed in ms since epoch at which the
version of the view was created. |
-| Required | summary | A string map summarizes the version changes, including
`operation`, described in [Summary](#summary). |
-| Required | representations | A list of "representations" as described in
[Representations](#representations). |
+| Requirement | Field name | Description |
+|-------------|-------------------|-------------|
+| _required_ | `version-id` | ID for the version |
+| _required_ | `timestamp-ms` | Timestamp when the version was created (ms
from epoch) |
+| _required_ | `summary` | A string to string map of [summary
metadata](#summary) about the version |
+| _required_ | `representations` | A list of
[representations](#representations) for the view definition |
-#### Version Log
+#### Summary
-Field “version-log” is an array of structs that describe when each version was
considered "current". Creation time is different and is stored in each
version's metadata. This allows you to reconstruct what someone would have seen
at some point in time. If the view has been updated and rolled back, this will
show it. The struct has the following fields:
+Summary is a string to string map of metadata about a view version. Common
metadata keys are documented here.
-| Required/Optional | Field Name | Description |
-|-------------------|------------|-------------|
-| Required | timestamp-ms | The timestamp when the referenced version was made
the current version |
-| Required | version-id | Version id of the view |
+| Requirement | Key | Value |
+|-------------|------------------|-------|
+| _required_ | `operation` | Operation that caused this metadata to be
created; must be `create` or `replace` |
+| _optional_ | `engine-name` | Name of the engine that created the view
version |
+| _optional_ | `engine-version` | Version of the engine that created the view
version |
-#### Summary
+#### Representations
-Field “summary” is a string map with the following keys. Only `operation` is
required. Engines may store additional key-value pairs in this map.
+View definitions can be represented in multiple ways. Representations are
documented ways to express a view definition.
-| Required/Optional | Key | Value |
-|-------------------|-----|-------|
-| Required | operation | A string value indicating the view operation that
caused this metadata to be created. Allowed values are “create” and “replace”. |
-| Optional | engine-version | A string value indicating the version of the
engine that performed the operation |
+A view version can have more than one representation. All representations for
a version must express the same underlying definition. Engines are free to
choose the representation to use.
-#### Representations
+Each representation is an object with at least one common field, `type`, that
is one of the following:
+* `sql`: a SQL SELECT statement that defines the view
+
+Representations further define metadata for each type.
+
+##### SQL representation
+
+The SQL representation stores the view definition as a SQL SELECT, with
metadata such as the SQL dialect.
+
+| Requirement | Field name | Type | Description |
+|-------------|---------------------|----------------|-------------|
+| _required_ | `type` | `string` | Must be `sql` |
+| _required_ | `sql` | `string` | A SQL SELECT statement |
+| _required_ | `dialect` | `string` | The dialect of the
`sql` SELECT statement (e.g., "trino" or "spark") |
+| _optional_ | `schema-id` | `int` | ID of the schema
produced by the SELECT statement |
+| _optional_ | `default-catalog` | `string` | Catalog name to use
when a reference in the SELECT does not contain a catalog |
+| _optional_ | `default-namespace` | `list<string>` | Namespace to use when a
reference in the SELECT is a single identifier |
+| _optional_ | `field-aliases` | `list<string>` | Column names optionally
specified in the create statement |
+| _optional_ | `field-comments` | `list<string>` | Column descriptions
(COMMENT) optionally specified in the create statement |
+
+For example:
+
+```sql
+USE prod.default
+```
+```sql
+CREATE OR REPLACE VIEW event_agg (
+ event_count COMMENT 'Count of events',
+ event_date) AS
+SELECT
+ COUNT(1), CAST(event_ts AS DATE)
+FROM events
+GROUP BY 2
+```
+
+This create statement would produce the following `sql` representation
metadata:
+
+| Field name | Value |
+|---------------------|-------|
+| `type` | `"sql"` |
+| `sql` | `"SELECT\n COUNT(1), CAST(event_ts AS DATE)\nFROM
events\nGROUP BY 2"` |
+| `dialect` | `"spark"` |
+| `schema-id` | `1` |
+| `default-catalog` | `"prod"` |
+| `default-namespace` | `["default"]` |
+| `field-aliases` | `["event_count", "event_date"]` |
+| `field-comments` | `["Count of events", null]` |
-Each representation is stored as an object with only one common field "type".
-The rest of the fields are interpreted based on the type.
+If a create statement does not include column names or comments before `AS`,
the fields should be omitted.
-##### Original View Definition in SQL
+#### Version log
-This type of representation stores the original view definition in SQL and its
SQL dialect.
+The version log tracks changes to the view's current version. This is the
view's history and allows reconstructing what version of the view would have
been used at some point in time.
-| Required/Optional | Field Name | Description |
-|-------------------|------------|-------------|
-| Required | type | A string indicating the type of representation. It is set
to "sql" for this type. |
-| Required | sql | A string representing the original view definition in SQL |
-| Required | dialect | A string specifying the dialect of the ‘sql’ field. It
can be used by the engines to detect the SQL dialect. |
-| Optional | schema-id | ID of the view's schema when the version was created |
-| Optional | default-catalog | A string specifying the catalog to use when the
table or view references in the view definition do not contain an explicit
catalog. |
-| Optional | default-namespace | The namespace to use when the table or view
references in the view definition do not contain an explicit namespace. Since
the namespace may contain multiple parts, it is serialized as a list of
strings. |
-| Optional | field-aliases | A list of strings of field aliases optionally
specified in the create view statement. The list should have the same length as
the schema's top level fields. See the example below. |
-| Optional | field-docs | A list of strings of field comments optionally
specified in the create view statement. The list should have the same length as
the schema's top level fields. See the example below. |
+Note that this is not the version's creation time, which is stored in each
version's metadata. A version can appear multiple times in the version log,
indicating that the view definition was rolled back.
-For `CREATE VIEW v (alias_name COMMENT 'docs', alias_name2, ...) AS SELECT
col1, col2, ...`,
-the field aliases are 'alias_name', 'alias_name2', and etc., and the field
docs are 'docs', null, and etc.
+Each entry in `version-log` is a struct with the following fields:
+
+| Requirement | Field name | Description |
+|-------------|----------------|-------------|
+| _required_ | `timestamp-ms` | Timestamp when the view's
`current-version-id` was updated (ms from epoch) |
+| _required_ | `version-id` | ID that `current-version-id` was set to |
## Appendix A: An Example
@@ -124,142 +167,164 @@ The JSON metadata file format is described using an
example below.
Imagine the following sequence of operations:
-* `CREATE TABLE base_tab(c1 int, c2 varchar);`
-* `INSERT INTO base_tab VALUES (1,’one’), (2,’two’);`
-* `CREATE VIEW common_view AS SELECT * FROM base_tab;`
-* `CREATE OR REPLACE VIEW common_view AS SELECT count(*) AS my_cnt FROM
base_tab;`
+```sql
+USE prod.default
+```
+```sql
+CREATE OR REPLACE VIEW event_agg (
+ event_count COMMENT 'Count of events',
+ event_date)
+COMMENT 'Daily event counts'
+AS
+SELECT
+ COUNT(1), CAST(event_ts AS DATE)
+FROM events
+GROUP BY 2
+```
+
-The metadata JSON file created at the end of step 3 looks as follows. The file
path looks like:
-`s3://my_company/my/warehouse/anorwood.db/common_view`
+The metadata JSON file created looks as follows.
-The path is intentionally similar to the path for iceberg tables and contains
a ‘metadata’ directory.
(`METASTORE_WAREHOUSE_DIR/<dbname>.db/<viewname>/metadata`)
+The path is intentionally similar to the path for Iceberg tables and uses a
`metadata` directory.
-The metadata directory contains View Version Metadata files. The text after
'=>' symbols describes the fields.
+```
+s3://bucket/warehouse/default.db/event_agg/metadata/00001-(uuid).metadata.json
+```
```
{
- "format-version" : 1, => JSON format. Will change as format evolves.
- "location" : "s3n://my_company/my/warehouse/anorwood.db/common_view",
- "current-version-id" : 1, => current / latest version of the view. ‘1’ here
since this metadata was created when the view was created.
- "properties" : { => shows properties of the view
- "comment" : "View captures all the data from the table" => View comment
+ "format-version" : 1,
+ "location" : "s3://bucket/warehouse/default.db/event_agg",
+ "current-version-id" : 1,
+ "properties" : {
+ "comment" : "Daily event counts"
},
- "versions" : [ { => Last few versions of the view.
+ "versions" : [ {
"version-id" : 1,
"timestamp-ms" : 1573518431292,
"summary" : {
- "operation" : "create", => View operation that caused this metadata to
be created
- "engineVersion" : "presto-350" => Version of the engine that performed
the operation (create / replace)
+ "operation" : "create",
+ "engine-name" : "Spark",
+ "engineVersion" : "3.3.2"
},
- "representations" : [ { => SQL metadata of the view
+ "representations" : [ {
"type" : "sql",
- "sql" : "SELECT *\nFROM\n base_tab\n", => original view SQL
- "dialect" : "presto",
+ "sql" : "SELECT\n COUNT(1), CAST(event_ts AS DATE)\nFROM
events\nGROUP BY 2",
+ "dialect" : "spark",
"schema-id" : 1,
- "default-catalog" : "iceberg",
- "default-namespace" : [ "anorwood" ]
+ "default-catalog" : "prod",
+ "default-namespace" : [ "default" ],
+ "field-aliases" : ["event_count", "event_date"],
+ "field-comments" : ["Count of events", null]
} ]
} ],
- "version-log" : [ { => Log of the created versions
- "timestamp-ms" : 1573518431292,
- "version-id" : 1
- } ],
- "schemas": [ { => Schema of the view expressed in Iceberg types
+ "current-schema-id": 1,
+ "schemas": [ {
"schema-id": 1,
"type" : "struct",
"fields" : [ {
- "id" : 0,
- "name" : "c1",
+ "id" : 1,
+ "name" : "col1",
"required" : false,
- "type" : "int",
- "doc" : "" => Column comment
+ "type" : "int"
}, {
- "id" : 1,
- "name" : "c2",
+ "id" : 2,
+ "name" : "col2",
"required" : false,
- "type" : "string",
- "doc" : ""
+ "type" : "date"
} ]
} ],
- "current-schema-id": 1
+ "version-log" : [ {
+ "timestamp-ms" : 1573518431292,
+ "version-id" : 1
+ } ]
}
```
-The Iceberg / view library creates a new metadata JSON file every time the
view undergoes a DDL change. This way the history of how the view evolved can
be maintained. Following metadata JSON file was created at the end of Step 4.
+Each change creates a new metadata JSON file.
+
+```sql
+USE prod.other_db;
+CREATE OR REPLACE VIEW default.event_agg (
+ event_count,
+ event_date)
+AS
+SELECT
+ COUNT(1), CAST(event_ts AS DATE)
+FROM prod.default.events
+GROUP BY 2
+```
+
+Updating the view produces a new metadata file that completely replaces the
old:
+```
+s3://bucket/warehouse/default.db/event_agg/metadata/00002-(uuid).metadata.json
+```
```
{
"format-version" : 1,
- "location" : "s3n://my_company/my/warehouse/anorwood.db/common_view",
- "current-version-id" : 2,
- "properties" : { => shows properties of the view
- "comment" : "View captures count of the data from the table"
+ "location" : "s3://bucket/warehouse/default.db/event_agg",
+ "current-version-id" : 1,
+ "properties" : {
+ "comment" : "Daily event counts"
},
"versions" : [ {
"version-id" : 1,
"timestamp-ms" : 1573518431292,
"summary" : {
"operation" : "create",
- "engineVersion" : "presto-350"
+ "engine-name" : "Spark",
+ "engineVersion" : "3.3.2"
},
"representations" : [ {
"type" : "sql",
- "sql" : "SELECT *\nFROM\n base_tab\n",
- "dialect" : "presto",
+ "sql" : "SELECT\n COUNT(1), CAST(event_ts AS DATE)\nFROM
events\nGROUP BY 2",
+ "dialect" : "spark",
"schema-id" : 1,
- "default-catalog" : "iceberg",
- "default-namespace" : [ "anorwood" ]
- } ],
- "properties" : { }
+ "default-catalog" : "prod",
+ "default-namespace" : [ "default" ],
+ "field-aliases" : ["event_count", "event_date"],
+ "field-comments" : ["Count of events", null]
+ } ]
}, {
"version-id" : 2,
- "timestamp-ms" : 1573518440265,
+ "timestamp-ms" : 1573518981593,
"summary" : {
- "operation" : "replace", => The ‘replace’ operation caused this latest
version creation
- "engineVersion" : "spark-2.4.4"
+ "operation" : "create",
+ "engine-name" : "Spark",
+ "engineVersion" : "3.3.2"
},
"representations" : [ {
"type" : "sql",
- "sql" : "SELECT \"count\"(*) my_cnt\nFROM\n base_tab\n", => Note the
updated text from the ‘replace’ view statement
+ "sql" : "SELECT\n COUNT(1), CAST(event_ts AS DATE)\nFROM
prod.default.events\nGROUP BY 2",
"dialect" : "spark",
- "schema-id" : 2,
- "default-catalog" : "iceberg",
- "default-namespace" : [ "anorwood" ]
+ "schema-id" : 1,
+ "default-catalog" : "prod",
+ "default-namespace" : [ "default" ],
+ "field-aliases" : ["event_count", "event_date"]
} ]
} ],
- "version-log" : [ {
- "timestamp-ms" : 1573518431292,
- "version-id" : 1
- }, {
- "timestamp-ms" : 1573518440265,
- "version-id" : 2
- } ],
- "schemas": [ { => Schema of the view expressed in Iceberg types
+ "current-schema-id": 1,
+ "schemas": [ {
"schema-id": 1,
"type" : "struct",
"fields" : [ {
- "id" : 0,
- "name" : "c1",
- "required" : false,
- "type" : "int",
- "doc" : "" => Column comment
- }, {
"id" : 1,
- "name" : "c2",
+ "name" : "col1",
"required" : false,
- "type" : "string",
- "doc" : ""
- } ]
- }, { => Schema change is reflected here
- "schema-id": 2,
- "type" : "struct",
- "fields" : [ {
- "id" : 0,
- "name" : "my_cnt",
+ "type" : "int"
+ }, {
+ "id" : 2,
+ "name" : "col2",
"required" : false,
- "type" : "long",
- "doc" : ""
+ "type" : "date"
} ]
} ],
- "current-schema-id": 2
+ "version-log" : [ {
+ "timestamp-ms" : 1573518431292,
+ "version-id" : 1
+ }, {
+ "timestamp-ms" : 1573518981593,
+ "version-id" : 2
+ } ]
}
```
