An external table is a Kusto schema entity that references data stored outside the Azure Data Explorer database. Similar to tables, an external table has a well-defined schema (an ordered list of column name and data type pairs). Unlike tables, data is stored and managed outside the cluster.
How to create Azure table?
Create a table
- Destination tab. The Create table window opens with the Destination tab selected. ...
- Source tab. In Source type, select the data source you'll use to create your table mapping. ...
- Edit Schema. In the Schema tab, your data format and compression are automatically identified in the left-hand pane.
- Create table completed window. ...
How are external tables created?
When you create an external table, you specify the following attributes:
- TYPE — specifies the type of external table. ...
- DEFAULT DIRECTORY — specifies the default directory to use for all input and output files that do not explicitly name a directory object. ...
- ACCESS PARAMETERS — describe the external data source and implement the type of external table that was specified. ...
What is Azure data table?
Azure Table is a service provided by Microsoft Azure to store structured NoSQL data in the cloud, data does not have schema and provides key or attribute to store data. This table is schema-less which offers flexibility and scalability to handle datasets like web application data, address books, etc. as service requires.
What is table storage in azure?
Table Storage Concepts
- Accounts: All access to Azure Storage is done through a storage account.
- Table: A table is a collection of entities.
- Entity: An entity is a set of properties, similar to a database row.
- Properties: A property is a name-value pair.
What is an external table?
An external table is a table whose data come from flat files stored outside of the database.
What is external table in Azure SQL Database?
The external table is created on Managed Instance, which with appropriate credentials can be used to query data from a matching table on SQL Database.
What is the use of external table?
External tables allow Oracle to query data that is stored outside the database in flat files. The ORACLE_LOADER driver can be used to access any data stored in any format that can be loaded by SQL*Loader. No DML can be performed on external tables but they can be used for query, join and sort operations.
What is external table in Polybase?
External Tables in SQL Server 2016 are used to set up the new Polybase feature with SQL Server. With this new feature (Polybase), you can connect to Azure blog storage or Hadoop to query non-relational or relational data from SSMS and integrate it with SQL Server relational tables.
Can we create external table in Azure SQL Database?
Overview: SQL Server This command creates an external table for PolyBase to access data stored in a Hadoop cluster or Azure blob storage PolyBase external table that references data stored in a Hadoop cluster or Azure blob storage. Use an external table with an external data source for PolyBase queries.
How do I create an external data source in Azure SQL Database?
Next stepsALTER EXTERNAL DATA SOURCE (Transact-SQL)CREATE DATABASE SCOPED CREDENTIAL (Transact-SQL)CREATE EXTERNAL FILE FORMAT (Transact-SQL)CREATE EXTERNAL TABLE (Transact-SQL)sys.external_data_sources (Transact-SQL)PolyBase Connectivity Configuration.
What is difference between external table and SQL Loader?
If you look at the External Table syntax, it looks suspiciously like SQL*Loader control file syntax :-) If your external table is going to be repeatedly used in multiple queries it might be faster to load a table (as you're doing now) rather than rescan your external table for each query.
Where are Azure synapse tables stored?
For Azure Synapse Analytics the table is stored on a distribution database on each Compute node. For Analytics Platform System (PDW), the table is stored in a SQL Server filegroup that spans the Compute node.
Why do we create external tables in hive?
External tables are an excellent way to manage data on the Hive since Hive does not have ownership of the data stored inside External tables. In case, if the user drops the External tables then only the metadata of tables will be removed and the data will be safe.
How do you create a external table?
To create an external table, you must have the CREATE EXTERNAL TABLE administration privilege and the List privilege on the database where you are defining the table. If the schema where you define the table is not the default schema, you must have the List privilege on the schema as well.
What is PolyBase in Azure?
PolyBase is a technology that accesses external data stored in Azure Blob storage or Azure Data Lake Store via the T-SQL language.
What is the difference between internal and external tables in hive?
To answer you Question : For External Tables, Hive stores the data in the LOCATION specified during creation of the table(generally not in warehouse directory). If the external table is dropped, then the table metadata is deleted but not the data. For Internal tables, Hive stores data into its warehouse directory.
How do you create a external table?
To create an external table, you must have the CREATE EXTERNAL TABLE administration privilege and the List privilege on the database where you are defining the table. If the schema where you define the table is not the default schema, you must have the List privilege on the schema as well.
What is the difference between internal and external tables in hive?
To answer you Question : For External Tables, Hive stores the data in the LOCATION specified during creation of the table(generally not in warehouse directory). If the external table is dropped, then the table metadata is deleted but not the data. For Internal tables, Hive stores data into its warehouse directory.
What is an external table in hive?
An external table is a table for which Hive does not manage storage. If you delete an external table, only the definition in Hive is deleted. The data remains. An internal table is a table that Hive manages. If you delete an internal table, both the definition in Hive and the data are deleted.
How do I find an external table in SQL Server?
Columnsschema_name - external table schema name.table_name - external table name.source_name - name of the data source in the current database.location - the connection string, which includes the protocol, IP address, and port for the external data source.source_type - data source type:More items...•
What is a native external table?
Native external tables that you can use to read and export data in various data formats such as CSV and Parquet. Native external tables are available in serverless SQL pools, and they are in public preview in dedicated SQL pools.
What are external tables in Hadoop?
Depending on the type of the external data source, you can use two types of external tables: Hadoop external tables that you can use to read and export data in various data formats such as CSV, Parquet, and ORC. Hadoop external tables are available in dedicated SQL pools, but they aren't available in serverless SQL pools.
What is a credential in Azure?
CREDENTIAL = <database scoped credential> is optional credential that will be used to authenticate on Azure storage. External data source without credential can access public storage account or use the caller's Azure AD identity to access files on storage.
Where to store Azure Blob data?
Import data from Azure Blob Storage and Azure Data Lake Storage and store it in a dedicated SQL pool (only Hadoop tables in dedicated pool).
Can data source access only files on Azure?
Data source can have a credential that enables external tables to access only the files on Azure storage using SAS token or workspace Managed Identity - For examples, see the Develop storage files storage access control article.
Can you query an external table in Synapse?
Using Data Lake exploration capabilities of Synapse Studio you can now create and query an external table using Synapse SQL pool with a simple right-click on the file. The one-click gesture to create external tables from the ADLS Gen2 storage account is only supported for Parquet files.
Introduction
The elastic database query feature in Azure SQL allows you to run t-SQL statements that incorporate tables from other Azure SQL databases, meaning that you are able to run queries that span multiple databases.
Conclusions
Running SELECT * has no errors, but new column is not part of the result-set. This is expected and confirms that that External table DDL is used to formulate the SELECT statement.
Conclusions
Running SELECT * has errors, because the deleted column is part of the External table DDL and is used to formulate the SELECT statement.
Where are external tables stored?
The data files for an external table are stored in Hadoop or Azure blob storage. These data files are created and managed by your own processes. It is your responsibility to manage the security of the external data.
Can an external table be changed?
Limitations and Restrictions. Since the data for an external table is not under the direct management control of SQL Server, it can be changed or removed at any time by an external process. As a result, query results against an external table aren't guaranteed to be deterministic.
What formats can you export external tables?
Using external table for export scenario is limited to the following formats: CSV, TSV, JSON and Parquet.
What command will fail if a table exists?
If the table exists, .create command will fail with an error. Use .create-or-alter or .alter to modify existing tables.
Can you iterate on all files referenced by an external table?
Iterating on all files referenced by an external table can be quite costly, depending on the number of files. Make sure to use limit parameter if you just want to see some URI examples.
Can an external table have the same name as a regular table?
An external table can't have the same name as a regular table in the same database. Schema. External data schema is described using the following format: ColumnName : ColumnType ColumnName : ColumnType ...] where ColumnName adheres to entity naming rules, and ColumnType is one of supported data types.
Why are native external tables better than external tables?
Native external tables have better performance when compared to external tables with TYPE=HADOOP in their external data source definition. This is because native external tables use native code to access external data. External tables are useful when you want to control access to external data in Synapse SQL pool.
Why use external tables in Synapse?
External tables are also useful if you want to use tools, such as Power BI , in conjunction with Synapse SQL pool. External tables can access two types of storage: Public storage where users access public storage files.
Where are CSV tables available?
Native CSV tables are currently available only in the serverless SQL pools.
Can you specify a pattern for a table?
You can specify the pattern that the files must satisfy in order to be referenced by the external table. The pattern is required only for Parquet and CSV tables. If you are using Delta Lake format, you need to specify just a root folder, and the external table will automatically find the pattern.
Can you create external tables in SQL Server?
You can create external tables that access data on an Azure storage account that allows access to users with some Azure AD identity or SAS key. You can create external tables the same way you create regular SQL Server external tables.
When to use external table in Kusto?
Use the external table to query the SQL table when the query requires reading the entire table (or relevant columns) for further execution on Kusto side. When an SQL query can be optimized in T-SQL, use the sql_request plugin.
What is the SQL query execution?
SQL external table query implementation will execute SELECT x, s FROM MySqlTable statement, where x and s are external table column names. The rest of the query will execute on the Kusto side.
What is partitioning external tables?
We strongly recommend partitioning your external tables, which requires that your underlying data is organized using logical paths that include date, time, country, or similar dimensions in the path . Partitioning divides your external table data into multiple parts using partition columns. Partition columns must evaluate as expressions that parse the path and/or filename information stored in the METADATA$FILENAME pseudocolumn. A partition consists of all data files that match the path and/or filename in the expression for the partition column.
Where is data stored in a table?
In a typical table, the data is stored in the database ; however, in an external table, the data is stored in files in an external stage. External tables store file-level metadata about the data files, such as the filename, a version identifier and related properties. This enables querying data stored in files in an external stage as if it were inside a database. External tables can access data stored in any format supported by COPY INTO <table> statements.
Why is materialized view better than external view?
In many cases, materialized views over external tables can provide performance that is faster than equivalent queries over the underlying external table. This performance difference can be significant when a query is run frequently or is sufficiently complex.
Can external tables be read only?
External tables can access data stored in any format supported by COPY INTO <table> statements. External tables are read-only, therefore no DML operations can be performed on them; however, external tables can be used for query and join operations. Views can be created against external tables. Querying data stored external to ...
Can views be created against external tables?
Views can be created against external tables. Querying data stored external to the database is likely to be slower than querying native database tables; however, materialized views based on external tables can improve query performance.
Introduction
- The elastic database query feature in Azure SQL allows you to run t-SQL statements that incorporate tables from other Azure SQL databases, meaning that you are able to run queries that span multiple databases. The elastic query feature allows you to perform cross-database queries to access remote tables and to connect BI tools (Excel, Power BI) to ...
Let’s Run Some Tests
- In this blog, we present a series of tests where we have already defined external tables in one Azure SQL database, with the external data source being another, different Azure SQL database. We then modify the source table’s structure in the other database without modifying the external table definition in our database, and we run 2 types of SELECT statements to observe what happ…
Adding A New Column in The Source Table Without Updating The External Table DDL
- SELECT * results:SELECT * FROM [SCHEMA].[EXTERNAL_TABLE_LINK];- Query results successfully returned but the new field is not included in the result-set. This is expected since the External table do...
- SELECT <col1>, <col2>, … results:SELECT <col1>, <col2> FROM [SCHEMA].[EXTERNAL_TABLE_LINK];- Query results successfully returned but the new field i…
- SELECT * results:SELECT * FROM [SCHEMA].[EXTERNAL_TABLE_LINK];- Query results successfully returned but the new field is not included in the result-set. This is expected since the External table do...
- SELECT <col1>, <col2>, … results:SELECT <col1>, <col2> FROM [SCHEMA].[EXTERNAL_TABLE_LINK];- Query results successfully returned but the new field is not included in the result-set. This is expecte...
Conclusions
- Running SELECT *has no errors, but new column is not part of the result-set. This is expected and confirms that that External table DDL is used to formulate the SELECT statement.
- Running SELECT <col1>, <col2>has no errors if new column is not included in the SELECT statement.
- Running SELECT <col1>, <col2>does have errors if new column is included in the SELECT sta…
- Running SELECT *has no errors, but new column is not part of the result-set. This is expected and confirms that that External table DDL is used to formulate the SELECT statement.
- Running SELECT <col1>, <col2>has no errors if new column is not included in the SELECT statement.
- Running SELECT <col1>, <col2>does have errors if new column is included in the SELECT statement. This is expected since the External table DDL is missing the reference.
Deleting A Column in The Source Table Without Updating The External Table DDL
- SELECT * results:SELECT * FROM [SCHEMA].[EXTERNAL_TABLE_LINK];- Msg 46823, Level 16, State 1, Line 4- Error retrieving data from <AZURE_SERVER_NAME>.database.windows.net.<AZURE_SOURCE_DATABASE>. ‘I...
- SELECT <col1>, <col2>, … results:SELECT <col1>, <col2> FROM [SCHEMA].[EXTERNAL_TABLE_LINK];If the deleted column is included in the SELECT:- Msg 4…
- SELECT * results:SELECT * FROM [SCHEMA].[EXTERNAL_TABLE_LINK];- Msg 46823, Level 16, State 1, Line 4- Error retrieving data from <AZURE_SERVER_NAME>.database.windows.net.<AZURE_SOURCE_DATABASE>. ‘I...
- SELECT <col1>, <col2>, … results:SELECT <col1>, <col2> FROM [SCHEMA].[EXTERNAL_TABLE_LINK];If the deleted column is included in the SELECT:- Msg 46823, Level 16, State 1, Line 4- Error retrieving d...
Overview: SQL Server
Permissions
- Requires these user permissions: 1. CREATE TABLE 2. ALTER ANY SCHEMA 3. ALTER ANY EXTERNAL DATA SOURCE 4. ALTER ANY EXTERNAL FILE FORMAT 5. CONTROL DATABASE Note, the login that creates the external data source must have permission to read and write to the external data source, located in Hadoop or Azure blob storage.
Error Handling
- While executing the CREATE EXTERNAL TABLE statement, PolyBase attempts to connect to the external data source. If the attempt to connect fails, the statement will fail and the external table won't be created. It can take a minute or more for the command to fail since PolyBase retries the connection before eventually failing the query.
General Remarks
- In ad-hoc query scenarios, such as SELECT FROM EXTERNAL TABLE, PolyBase stores the rows that are retrieved from the external data source in a temporary table. After the query completes, PolyBase removes and deletes the temporary table. No permanent data is stored in SQL tables. In contrast, in the import scenario, such as SELECT INTO FROM EXTERNAL TABLE, PolyBase store…
Limitations and Restrictions
- Since the data for an external table is not under the direct management control of SQL Server, it can be changed or removed at any time by an external process. As a result, query results against an external table aren't guaranteed to be deterministic. The same query can return different results each time it runs against an external table. Similarly, a query might fail if the external dat…
Security
- The data files for an external table are stored in Hadoop or Azure blob storage. These data files are created and managed by your own processes. It is your responsibility to manage the security of the external data.
Examples
- A. Create an external table with data in text-delimited format
This example shows all the steps required to create an external table that has data formatted in text-delimited files. It defines an external data source mydatasource and an external file format myfileformat. These database-level objects are then referenced in the CREATE EXTERNAL TABL… - B. Create an external table with data in RCFile format
This example shows all the steps required to create an external table that has data formatted as RCFiles. It defines an external data source mydatasource_rc and an external file format myfileformat_rc. These database-level objects are then referenced in the CREATE EXTERNAL TA…
Next Steps
- Learn more about related concepts in the following articles: 1. CREATE EXTERNAL DATA SOURCE 2. CREATE EXTERNAL FILE FORMAT