Table of Contents
What are virtual columns in Oracle?
A virtual column is a table column whose values are calculated automatically using other column values, or another deterministic expression.
Can we create index on virtual column in Oracle?
Virtual columns are not supported for index-organized, external, object, cluster, or temporary tables. The expression used in the virtual column definition has the following restrictions: It cannot refer to another virtual column by name. It can only refer to columns defined in the same table.
What is virtual table in Oracle?
The term “virtual tables” is often used as a synonym for “fixed tables”, which are internal Oracle’s table-like structures, persistent only in SGA. They are used by the Oracle server engine for various things, and except that they are not physical tables, they behave much like ordinary tables.
Which are virtual tables?
A virtual table is an object that presents an SQL table interface but which is not stored in the database file, at least not directly. The virtual table mechanism is a feature of SQLite that allows SQLite to access and manipulate resources other than bits in the database file using the powerful SQL query language.
Why views are called virtual tables?
Virtual tables mean the tuples in views do not have physical existence and are not stored into the database. The tuples are like temporary data created as an outcome of the SQL query which typically draws filtered data from one or more base tables.
What is Pragma UDF?
The UDF pragma tells the compiler that the PL/SQL unit is a user defined function that is used primarily in SQL statements, which might improve its performance.
Which is not cursor attribute?
Cursor attributes (PL/SQL)
| Cursor attribute | %ISOPEN | %ROWCOUNT |
|---|---|---|
| Before OPEN | False | Cursor not open exception |
| After OPEN and before 1st FETCH | True | 0 |
| After 1st successful FETCH | True | 1 |
| After n th successful FETCH (last row) | True | n |
How many virtual tables are created?
Because there are 3 classes here, the compiler will set up 3 virtual tables: one for Base, one for D1, and one for D2. When a class object is created, *__vptr is set to point to the virtual table for that class. For example, when an object of type Base is created, *__vptr is set to point to the virtual table for Base.
What is virtual table in DBMS?
What is the with clause in Oracle?
Oracle With Clause is similar to temporary tables, where you store the data once and read it multiple times in your sql query. Oracle With Clause is used when a sub-query is executed multiple times. In simple With Clause is used to simply the complex SQL.
What is cur execute?
A cursor is an object which helps to execute the query and fetch the records from the database. The cursor plays a very important role in executing the query. This article will learn some deep information about the execute methods and how to use those methods in python.
How many rows a cursor can hold in Oracle?
there is no limit (i’m aware of) to the number of rows a cursor can process. Obviously, if you try and process 50 million records from a cursor, your program won’t likely finish anytime soon.
Where are virtual tables stored?
Vtables themselves are generally stored in the static data segment, as they are class-specific (vs. object-specific).
What is the need of virtual table?
At compile time, the compiler can’t know which code is going to be executed by the o->f() call since it doesn’t know what o points to. Hence, you need something called a “virtual table” which is basically a table of function pointers.
Why view is called virtual table?
SQL views are nothing but virtual tables reside in memory derived from one or more base tables. Virtual tables mean the tuples in views do not have physical existence and are not stored into the database.
Why with clause is faster in Oracle?
The answer as usual is DEPENDS on WHAT YOU ARE COMPARING with. If the WITH clause is used to do something that would otherwise need PL/SQL then it will be better than PL/SQL. The optimizer decides execution plan. And at times have no performance penalty, but at other times it may do something more complex.
What are virtual columns in Oracle Database 11g?
Virtual Columns in Oracle Database 11g Release 1. When queried, virtual columns appear to be normal table columns, but their values are derived rather than being stored on disc. The syntax for defining a virtual column is listed below.
What are the limitations of a virtual column in Oracle?
The virtual column cannot be an Oracle-supplied datatype, a user-defined type, or LOB or LONG RAW. The expression in the virtual column has the following restrictions: It cannot refer to other virtual columns. It cannot refer to normal columns of other tables.
How to create a virtual column in the table definition?
In 11g you can create a virtual column in the table definition. create table ord_tbl ( ord_id number(10) primary key, ord_name varchar2(20), quantity number(20), cost_per_item number(30), total_cost as (quantity*cost_per_item), ord_date date )
Are virtual columns supported for index-organized tables?
Virtual columns are not supported for index-organized, external, object, cluster, or temporary tables. It cannot refer to another virtual column by name. It can only refer to columns defined in the same table.