What is index unique scan?
INDEX UNIQUE SCAN. The INDEX UNIQUE SCAN performs the tree traversal only. The Oracle database uses this operation if a unique constraint ensures that the search criteria will match no more than one entry. TABLE ACCESS BY INDEX ROWID. The TABLE ACCESS BY INDEX ROWID operation retrieves the row from the table.
What does index range scan do?
During an index range scan, Oracle accesses adjacent index entries and then uses the ROWID values in the index to retrieve the table rows. An index range scan is used when the SQL statement contains a restrictive clause that requires a sequential range of values that are indexes for the table.
What is skip scan index in Oracle?
The index skip scan is a new execution plan in Oracle 10g whereby an Oracle query can bypass the leading-edge of a concatenated index and access the inside keys of a multi-values index.
What is unique scan in Oracle?
Answer: In an index unique scan, oracle reads the index nodes down to the leaf node level and them returns the ROWID for the appropriate single row from the calling SQL. Here is a report that lists index unique scans, which occur when the Oracle database engine uses an index to retrieve a specific row from a table.
Which query will result in index range scan?
Any query that does not use every column in the concatenated index with an = comparison will result in a range scan.
What is index fast full scan?
Fast full index scans are an alternative to a full table scan when the index contains all the columns that are needed for the query, and at least one column in the. index key has the NOT NULL constraint. A fast full scan accesses the data in the index itself, without accessing the table.
What is Skip index and how is it used?
Rather than restricting the search path using a predicate from the statement, Skip Scans are initiated by probing the index for distinct values of the prefix column. Each of these distinct values is then used as a starting point for a regular index search.
What is index full scan?
Index full scan is a mechanism where Oracle does not read all the required entries from the index by traversing the tree from top to leaf for the rows. Instead, it traverses the tree from top to bottom on “the left side” of the index.
Is full index scan bad?
While a covering index (seen with EXPLAIN as Extra: Using index ) is a very interesting performance optimization, a full index scan ( type: index ) is according to the documentation the 2nd worst possible execution plan after a full table scan.
How to do a unique scan on an index?
Index unique scan This is a method for looking up a single key value via a unique index and it always returns a single value. You must supply AT LEAST the leading column of the index to access data via the index (however this may return > 1 row as the uniqueness will not be guaranteed).
When to use range scan or unique scan?
Index range scan Index range scan is a method for accessing a range values of a particular column. AT LEAST the leading column of the index must be supplied to access data via the index. It can be used for range operations (e.g. > < >= <= between ) or where the data to be returned is not unique.
How is a range scan used in index lookup?
Index range scan Index range scan is a method for accessing a range values of a particular column. AT LEAST the leading column of the index must be supplied to access data via the index. AT LEAST the leading column of the index must be supplied to access data via the index.
When to use an index skip or full table scan?
However, in this situation the optimizer may choose to use an index skip scan. The query is unselective. If the optimizer determines that the query requires most of the blocks in the table, then it uses a full table scan, even though indexes are available. Full table scans can use larger I/O calls.