in oracle when i make a join, the process : Oracle take a main table from DB(in HDD) and using 'on condition' to take datas from sub table. is it right? and I'm wondering oracle check the condition at memory or at hdd?

When you execute a join in Oracle, the database will typically load the relevant data into memory and perform the join operation there. This is known as a "memory-based join" or "in-memory join."

During the join operation, Oracle will use the "ON" condition to compare the values of the specified columns in the main table and the sub table. If the condition is true, the relevant rows will be combined to form the result set.

However, it's important to note that the actual process of retrieving the data from disk and loading it into memory will depend on a number of factors, including the size of the tables, the available system resources, and the specific configuration of your Oracle instance.

In general, Oracle will try to optimize the join process by using various techniques such as indexing, caching, and parallel processing. The goal is to minimize the amount of disk I/O and maximize the use of memory to improve performance.

So to summarize, Oracle will load the relevant data into memory and perform the join operation there, using the "ON" condition to compare the values of the specified columns. However, the actual process of retrieving the data from disk and loading it into memory will depend on various factors and may be optimized using different techniques.

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so oracle cannot work on disk, just take datas from disk and every work using data is happend on memory right?

Oracle is a database management system that is designed to work with data stored both in memory and on disk. When you query data from an Oracle database, the system will read the relevant data from disk and load it into memory as needed to perform the operation.

Once the data is in memory, Oracle will typically perform the requested operation (such as a join) using the data in memory rather than going back to disk for every operation. This is because reading from memory is typically much faster than reading from disk, so it can help improve performance.

However, it's important to note that Oracle also uses various techniques such as caching and buffering to optimize disk I/O and improve performance. For example, it may cache frequently used data in memory to avoid the need to go back to disk for every operation. Or it may use a buffer cache to temporarily store data in memory while it's being written to or read from disk.

So to answer your question, Oracle does work with data stored on disk, but it also tries to minimize the need to go back to disk by using various optimization techniques to keep frequently accessed data in memory.

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than where is "cache" and "buffer" physically

In Oracle, both the cache and buffer are typically located in the main memory (RAM) of the server on which the database is running. When data is read from disk, it is typically loaded into a buffer cache in memory, and then subsequent reads of the same data will be served from the cache instead of going back to disk.

The buffer cache is a region of memory that is managed by the Oracle instance, and is used to store frequently accessed data in memory. The size of the buffer cache can be configured by the database administrator, and is typically set to a value that allows the most frequently accessed data to be cached in memory.

The data in the buffer cache is organized into data blocks, which are the smallest unit of data that can be read from or written to disk. When a query is executed, Oracle will typically read the relevant data blocks from the buffer cache in memory, and perform any necessary operations (such as a join) using the data in memory.

The cache, on the other hand, can refer to a few different things in the context of Oracle. It may refer to the result cache, which is a feature that allows frequently used query results to be cached in memory for improved performance. It may also refer to other types of caches used by Oracle, such as the shared pool and the data dictionary cache, which are used to store metadata and other information about the database in memory for quick access.

In general, the use of memory caches and buffers is a key technique used by Oracle (and other database management systems) to improve performance by reducing the need to access data from disk.