If the database optimizer selects the full table scan access strategy, the table is read sequentially. Index blocks do not need to be read.
For a full table scan, the read table blocks are added to the end of an LRU list. Therefore, no data blocks are forced out of the data buffer. As a result, in order to process a full table scan, comparatively little memory space is required within the data buffer.
The full table scan access strategy is very effective if a large part of a table (for example, 5% of all table records) needs to be read. In the above example, the full table scan access strategy is not effective, because only a few table records are required, but many table blocks need to be read.
In the concatenation access strategy, one index is reused. Therefore, various index search strings also exist. An index unique scan or an index range scan can be performed for the various index search strings. Duplicate entries in the resulting set are filtered out when the search results are concatenated.
In the SQL statement above, a WHERE condition with an IN operation is specified over field VBELN. The fields MANDT and VBELN are shown on the left of the primary index. Various index search strings are created, and an index range scan is performed over the primary index for each index search string. Finally, the result is concatenated (combined).
Index unique scan: The index selected is unique (primary index or unique secondary index) and specified fully. One or no table record is returned. This type of access is very effective, because a maximum of four data blocks need to be read.
Index range scan: The index selected is unique or non-unique. In the case of a unique index, not all index fields are specified in the WHERE clause. A range of the index is read and checked. An index range scan may not be as effective as a full table scan. The table records returned can range from none to all.
Full table scan: The whole table is read sequentially. Each table block is read once. Since no index is used, no index blocks are read. The table records returned can range from none to all.
Concatenation: An index is used more than once. Various areas of the index are read and checked. To ensure that the application reads each table record only once, the search results are concatenated, and duplicate entries are eliminated. The table records returned can range from none to all.
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