You can also do this techniques with NULL numeric values. I was reading "Indexing NULL table column values for fast SQL performance" article, where you have mentioned that if the column is having null values of very less amount and you want to select where column is null then to use the index, create function based index and changed your query to use that index.
Search BC Oracle Sites. If you like Oracle tuning, see the book " Oracle Tuning: The Definitive Reference ", with pages of tuning tips and scripts. Learn more. Index for nullable column Ask Question. Asked 9 years, 11 months ago.
Active 9 years, 11 months ago. Viewed 61k times. My question is simple: why there aren't null entries in indexes? Whether you had a Null-value index or not wouldn't matter the optimiser would have opted for a FTS for this query. This why I wrote that I tried it again with a hint and it didn't help Bitmap indexes will index nulls.
Of course, if you select any columns that can't be served just by the index you will end up with a FTS anyway unless you force a hint, in which case it will use your index but you will actually have worse IO performance — rejj. Ollie -- if the index is on ename , which is the implication, it would be entirely possible for the optimizer to choose a full index scan instead of a full table scan, if not for the issue with nulls. Show 1 more comment. Active Oldest Votes.
Please refer to "Restriction on Index Columns". All functions must be specified with parentheses, even if they have no parameters. Otherwise Oracle Database interprets them as column names. Indexes on character data are created in ascending or descending order of the character values in the database character set. Oracle Database treats descending indexes as if they were function-based indexes.
As with other function-based indexes, the database does not use descending indexes until you first analyze the index and the table on which the index is defined. Ascending unique indexes allow multiple NULL values. However, in descending unique indexes, multiple NULL values are treated as duplicate values and therefore are not permitted. Restriction on Ascending and Descending Indexes You cannot specify either of these clauses for a domain index. You cannot specify DESC for a reverse index.
If you omit this clause, then Oracle Database creates the index in the default tablespace of the owner of the schema containing the index. New partitions or subpartitions added to the local index will be created in the same tablespace s as the corresponding partitions or subpartitions of the underlying table. Use integer to specify the prefix length number of prefix columns to compress. For unique indexes, the valid range of prefix length values is from 1 to the number of key columns minus 1.
The default prefix length is the number of key columns minus 1. For nonunique indexes, the valid range of prefix length values is from 1 to the number of key columns. The default prefix length is the number of key columns. Oracle Database compresses only nonpartitioned indexes that are nonunique or unique indexes of at least two columns. This is the default. You can specify NOSORT to indicate to the database that the rows are already stored in the database in ascending order, so that Oracle Database does not have to sort the rows when creating the index.
If the rows of the indexed column or columns are not stored in ascending order, then the database returns an error. For greatest savings of sort time and space, use this clause immediately after the initial load of rows into a table. If you specify neither of these keywords, then SORT is the default. Restrictions on Reverse Indexes Reverse indexes are subject to the following restrictions:. If index is nonpartitioned, then this clause specifies the logging attribute of the index.
If index is partitioned, then this clause determines:. If you omit this clause, then the logging attribute is that of the tablespace in which it resides. Restrictions on Online Index Building Online index building is subject to the following restrictions:. Parallel DML is not supported during online index building. For a nonunique secondary index on an index-organized table, the number of index key columns plus the number of primary key columns that are included in the logical rowid in the index-organized table cannot exceed The logical rowid excludes columns that are part of the index key.
This clause has been deprecated. Oracle Database now automatically collects statistics during index creation and rebuild. This clause is supported for backward compatibility and will not cause errors.
Index Partitioning Clauses. The storage of partitioned database entities in tablespaces of different block sizes is subject to several restrictions. Please refer to Oracle Database Administrator's Guide for a discussion of these restrictions. By default, nonpartitioned indexes are global indexes. You can partition a global index by range or by hash.
In both cases, you can specify up to 32 columns as partitioning key columns. The partitioning column list must specify a left prefix of the index column list. Stickers, coasters, books and coffee mugs.
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