Hybrid Columnar Compression uses a logical construct called a compression unit to store a set of rows.
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When you load data into a table, the database stores groups of rows in columnar format, with the values for each column stored and compressed together. After the database has compressed the column data for a set of rows, the database fits the data into the compression unit.
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For example, you apply Hybrid Columnar Compression to a daily_sales table. At the end of every day, you populate the table with items and the number sold, with the item ID and date forming a composite primary key. The following table shows a subset of the rows in daily_sales.
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Table 2-2 Sample Table daily_sales
| Item_ID | Date | Num_Sold | Shipped_From | Restock |
|---|---|---|---|---|
| 1000 | 01-JUN-18 | 2 | WAREHOUSE1 | Y |
| 1001 | 01-JUN-18 | 0 | WAREHOUSE3 | N |
| 1002 | 01-JUN-18 | 1 | WAREHOUSE3 | N |
| 1003 | 01-JUN-14 | 0 | WAREHOUSE2 | N |
| 1004 | 01-JUN-18 | 2 | WAREHOUSE1 | N |
| 1005 | 01-JUN-18 | 1 | WAREHOUSE2 | N |
Assume that this subset of rows is stored in one compression unit. Hybrid Columnar Compression stores the values for each column together, and then uses multiple algorithms to compress each column. The database chooses the algorithms based on a variety of factors, including the data type of the column, the cardinality of the actual values in the column, and the compression level chosen by the user.
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As shown in the following graphic, each compression unit can span multiple data blocks. The values for a particular column may or may not span multiple blocks.
Figure 2-4 Compression Unit
Description of “Figure 2-4 Compression Unit”
If Hybrid Columnar Compression does not lead to space savings, then the database stores the data in the DBMS_COMPRESSION.COMP_BLOCK format. In this case, the database applies OLTP compression to the blocks, which reside in a Hybrid Columnar Compression segment.
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