How a select query works in Oracle?

When Oracle receives a sql query, it requires to run some pre-tasks before actually being able to really run the query. Combination of these tasks is called parsing. During parsing the below operations used to perform. Database validate the semantic of the statement.It checks whether a statement is meaningful or not.

When SCN number is generated in Oracle?

When a transaction commits, the database records an SCN for this commit. Oracle Database increments SCNs in the system global area (SGA). When a transaction modifies data, the database writes a new SCN to the undo data segment assigned to the transaction.

What is query SCN?

SCN (System Change Number) is nothing more than a stamp (number) that defines a committed version of a database at a point in time. This means that every committed transaction is assigned a unique SCN.

What happens when select query is executed?

Once the query execution plan is ready and available to execute, SQL Server storage engines gets the query plan and executes it based on the actual query plan. After executing the query, the desired output is returned to you.

How select query works internally?

The database will authenticate the user and if successful then a server process will be initiated on the server side and user process will be initiated on the client side. After this a valid session is establish between the client and the server. The user types a query on the sql prompt.

How do I find my SCN number?

How do I find my Scottish Candidate Number (SCN)? Your Scottish Candidate Number (SCN) is displayed directly under your name on your certificate. Alternatively, call us on 0345 279 1000 and we will confirm your details then provide you with your SCN Number.

How do I get a SCN for a specific time of a database?

HOW TO CONVERT SCN TO TIMESTAMP AND TIMESTAMP INTO SCN

1. Check the current scn of the database using below query. SQL>select current_scn from v$database;
2. To get the timestamp value from the current scn number. SQL> select scn_to_timestamp(4426538972) as timestamp from dual;
3. To get the scn number from the timestamp.

How do I get a SCN?

How do I know if my SCN is standby?

Check CURRENT SCN of the Oracle Database

1. Fetch the current SCN number in v$database view. Select CURRENT_SCN from v$database; SQL> Select CURRENT_SCN from v$database;
2. Get from flashback package. select dbms_flashback.get_system_change_number from dual;
3. Get the SCN from timestamp_to_scn function.

What happens when we execute a query in Oracle?

Execute Phase – During the execute phase, Oracle executes the statement, reports any possible errors, and if everything is as it should be, forms the result set. Unless the SQL statement being executed is a query, this is the last step of the execution.

What executes first in SQL query?

SQL’s from clause selects and joins your tables and is the first executed part of a query. This means that in queries with joins, the join is the first thing to happen.