ASP.NET Web PDF Document Viewer/Editor Control Library

And then run a query that could make use of this index on Y but only if Y is NOT NULL as in following query: ops$tkyte%ORA11GR2> select count(*) from t; Execution Plan ---------------------------------------------------------Plan hash value: 995313729 -----------------------------------------------------------------| Id | Operation | Name | Rows | Cost (%CPU)| Time | -----------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 1 (0)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | | | | 2 | INDEX FULL SCAN| T_IDX | 45 | 1 (0)| 00:00:01 | -----------------------------------------------------------------you would be happy to see the optimizer chose to use the small index on Y to count the rows rather than to full scan the entire table T However, if you drop that index and index column X instead ops$tkyte%ORA11GR2> drop index t_idx; Index dropped ops$tkyte%ORA11GR2> create index t_idx on t(x); Index created..

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and then run the query to count the rows once more, you would discover that the database does not, in fact cannot, use your index: ops$tkyte%ORA11GR2> select count(*) from t; Execution Plan ---------------------------------------------------------Plan hash value: 2966233522 ------------------------------------------------------------------| Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 3 (0)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | | | | 2 | TABLE ACCESS FULL| T | 45 | 3 (0)| 00:00:01 | ------------------------------------------------------------------It full-scanned the table It had to full-scan the table in order to count the rows This is due to the fact that in an Oracle B*Tree index, index key entries that are entirely null are not made.

Applications communicate with relational databases using Structured Query Language (SQL). Each time you create tables, create relationships, insert new records, or update or delete existing ones, you are explicitly or implicitly issuing SQL statements to the database. The examples in this chapter use a dialect of Standard SQL, called Transact-SQL (T-SQL), used by SQL Server and SQL Server Express. SQL has syntax to define the structure of a database schema (loosely speaking, a collection of data tables and their relations) and also syntax to manage the data within. These subsets of SQL are called Data Definition Language (DDL) and Data Manipulation Language (DML), respectively. The most important DDL statements are the following: CREATE/ALTER/DROP TABLE CREATE/DROP VIEW

That is, the index will not contain an entry for any row in the table T such that all of the columns in the index are null Since X is allowed to be null temporarily, the optimizer has to assume that X might be null and therefore would not be in the index on X Hence a count returned from the index might be different (wrong) from a count against the table We can see that if X had a non-deferrable constraint placed on it, this limitation is removed; that is, column X is in fact as good as column Y if the NOT NULL constraint is not deferrable: ops$tkyte%ORA11GR2> alter table t drop constraint x_not_null; Table altered ops$tkyte%ORA11GR2> alter table t modify x constraint x_not_null not null; Table altered.

ops$tkyte%ORA11GR2> set autotrace traceonly explain ops$tkyte%ORA11GR2> select count(*) from t; Execution Plan ---------------------------------------------------------Plan hash value: 995313729 -----------------------------------------------------------------| Id | Operation | Name | Rows | Cost (%CPU)| Time | -----------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 1 (0)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | | | | 2 | INDEX FULL SCAN| T_IDX | 45 | 1 (0)| 00:00:01 | -----------------------------------------------------------------So, the bottom line is, only use deferrable constraints where you have an identified need to use them They introduce subtle side effects that could cause differences in your physical implementation (non-unique vs unique indexes) or in your query plans as just demonstrated!.

ADO.NET is the central database access machinery in the .NET Framework, and it provides full XML support, disconnected and typed datasets, scalability, and high performance. In this section, we give a brief overview of the ADO.NET fundamentals.