Cratedb Generated Column Primary Key

Posted on 11.04.2020by admin

Primary Key Generation Using Oracle's Sequence

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Cratedb Generated Column Primary Key Excel

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Oracle provides the sequence utility to automatically generate unique primary keys. To use this utility to auto-generate primary keys for a CMP entity bean, you must create a sequence table and use the @AutomaticKeyGeneration annotation to point to this table.

In your Oracle database, you must create a sequence table that will create the primary keys, as shown in the following example:

This creates a sequences of primary key values, starting with 1, followed by 2, 3, and so forth. The sequence table in the example uses the default increment 1, but you can change this by specifying the increment keyword, such as increment by 3. When you do the latter, you must specify the exact same value in the cacheSize attribute of the @AutomaticKeyGeneration annotation:

If you have specified automatic table creation in the CMP bean's project settings, the sequence table will be created automatically when the entity bean is deployed. For more information, see @JarSettings Annotation. For more information on the definition of a CMP entity bean, see below.

Primary Key Generation Using SQL Server's IDENTITY

In SQL Server you can use the IDENTITY keyword to indicate that a primary-key needs to be auto-generated. The following example shows a common scenario where the first primary key value is 1, and the increment is 1:

In the CMP entity bean definition you need to specify SQLServer(2000) as the type of automatic key generator you are using. You can also provide a cache size:

Primary Key Generation Using a Named Sequence Table

A named sequence table is similar to the Oracle sequence functionality in that a dedicated table is used to generate primary keys. However, the named sequence table approach is vendor-neutral. To auto-generate primary keys this way, create a named sequence table using the two SQL statements shown in the example:

In the CMP entity bean definition you need to specify the named sequence table as the type of automatic key generator you are using. You can also provide a cache size:

Note. When you specify a cacheSize value for a named sequence table, a series of unique values are reserved for entity bean creation. When a new cache is necessary, a second series of unique values is reserved, under the assumption that the first series of unique values was entirely used. This guarantees that primary key values are always unique, although it leaves open the possibility that primary key values are not necessarily sequential. For instance, when the first series of values is 10..20, the second series of values is 21-30, even if not all values in the first series were actually used to create entity beans.

Defining the CMP Entity Bean

When defining a CMP entity bean that uses one of the primary key generators, you use the the @AutomaticKeyGeneration annotation to point to the name of the primary key generator table to obtain primary keys. Also, you must define a primary key field of type Integer or Long to set and get the auto-generated primary key. However, the ejbCreate method does not take a primary key value as an argument. Instead the EJB container adds the correct primary key to the entity bean record.

The following example shows what the entity bean might look like. Notice that the bean uses the named sequence option described above, and that ejbCreate Generated

method does not take a primary key:

What's new

SQL Server 2016 (13.x) introduces these features for columnstore performance enhancements:

Always On supports querying a columnstore index on a readable secondary replica.
Multiple Active Result Sets (MARS) supports columnstore indexes.
A new dynamic management view sys.dm_db_column_store_row_group_physical_stats (Transact-SQL) provides performance troubleshooting information at the row group level.
Single-threaded queries on columnstore indexes can run in batch mode. Previously, only multi-threaded queries could run in batch mode.
The SORT operator runs in batch mode.
Multiple DISTINCT operation runs in batch mode.
Window Aggregates now runs in batch mode for database compatibility level 130 and higher.
Aggregate Pushdown for efficient processing of aggregates. This is supported on all database compatibility levels.
String predicate pushdown for efficient processing of string predicates. This is supported on all database compatibility levels.
Snapshot isolation for database compatibility level 130 and higher.

Improve performance by combining nonclustered and columnstore indexes

Starting with SQL Server 2016 (13.x), you can define nonclustered indexes on a clustered columnstore index.

Example: Improve efficiency of table seeks with a nonclustered index

To improve efficiency of table seeks in a data warehouse, you can create a nonclustered index designed to run queries that perform best with table seeks. For example, queries that look for matching values or return a small range of values will perform better against a B-tree index rather than a columnstore index. They don't require a full table scan through the columnstore index and will return the correct result faster by doing a binary search through a B-tree index.

Example: Use a nonclustered index to enforce a primary key constraint on a columnstore table

By design, a columnstore table does not allow a clustered primary key constraint. Now you can use a nonclustered index on a columnstore table to enforce a primary key constraint. A primary key is equivalent to a UNIQUE constraint on a non-NULL column, and SQL Server implements a UNIQUE constraint as a nonclustered index. Combining these facts, the following example defines a UNIQUE constraint on the non-NULL column accountkey. The result is a nonclustered index that enforces a primary key constraint as a UNIQUE constraint on a non-NULL column.

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Next, the table is converted to a clustered columnstore index. During the conversion, the nonclustered index persists. The result is a clustered columnstore index with a nonclustered index that enforces a primary key constraint. Since any update or insert on the columnstore table will also affect the nonclustered index, all operations that violate the unique constraint and the non-NULL will cause the entire operation to fail.

The result is a columnstore index with a nonclustered index that enforces a primary key constraint on both indexes.

Improve performance by enabling row-level and row-group-level locking

To complement the nonclustered index on a columnstore index feature, SQL Server 2016 (13.x) offers granular locking capability for select, update, and delete operations. Queries can run with row-level locking on index seeks against a nonclustered index and rowgroup-level locking on full table scans against the columnstore index. Use this to achieve higher read/write concurrency by using row-level and rowgroup-level locking appropriately.

Snapshot isolation and read-committed snapshot isolations

Use snapshot isolation (SI) to guarantee transactional consistency, and read-committed snapshot isolations (RCSI) to guarantee statement level consistency for queries on columnstore indexes. This allows the queries to run without blocking data writers. This non-blocking behavior also significantly reduces the likelihood of deadlocks for complex transactions. For more information, see Snapshot Isolation in SQL Server on MSDN.

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