DML and Hybrid Columnar Compression

Hybrid Columnar Compression has implications for row locking in different types of DML operations.

Direct Path Loads and Conventional Inserts

When loading data into a table that uses Hybrid Columnar Compression, you can use either conventional inserts or direct path loads. Direct path loads lock the entire table, which reduces concurrency.

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Oracle Database 12c Release 2 (12.2) adds support for conventional array inserts into the Hybrid Columnar Compression format. The advantages of conventional array inserts are:

• Inserted rows use row-level locks, which increases concurrency.
• Automatic Data Optimization (ADO) and Heat Map support Hybrid Columnar Compression for row-level policies. Thus, the database can use Hybrid Columnar Compression for eligible blocks even when DML activity occurs on other parts of the segment.

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When the application uses conventional array inserts, Oracle Database stores the rows in compression units when the following conditions are met:

• The table is stored in an ASSM tablespace.
• The compatibility level is 12.2.0.1 or later.
• The table definition satisfies the existing Hybrid Columnar Compression table constraints, including no columns of type LONG, and no row dependencies.

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Conventional inserts generate redo and undo. Thus, compression units created by conventional DML statement are rolled back or committed along with the DML. The database automatically performs index maintenance, just as for rows that are stored in conventional data blocks.

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