Oracle Learning SQL statement Execution order

1. Order of execution:

Client input SQL statement

SQL statements reach the DB instance over the network

After the server porcess receives the SQL statement

A. Parse the SQL statement into an execution plan before executing

B, parsing: Consume a lot of resources: CPU, IO, if there is a cache in the shared pool does not need to parse again.

(Shared pool: Cached SQL statements and execution plans for SQL statements)

(Buffer case: Cache DBF Data

Fetch data procedure: Server porcess First checks if there is data in the buffer case, and if there is data, directly

The data in the buffer catch is returned to the user (logical read); if not, enter DBF to fetch the data to the buffer catch

(physical read).

Hit rate: logical read/(logical read + physical read)

Low hit rate: inefficient and concerned about efficiency should also look at the number of physical reads per second.

2.smon is responsible for maintaining the internal database, Pmon the server Proces.

3.buffer Cath Status:

Connected (server process/pin when reading or writing data), clean, unused, dirty (requires write, cached data is consistent with database data after write, cache becomes clean), cache order is used: not used, clean and dirty

Oracle Learning SQL statement Execution order