SQL Row Source Generation

 The row source generator is software that receives the optimal execution plan from the optimizer and produces an iterative plan, called the query plan, that is usable by the rest of the database.

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The query plan takes the form of a combination of steps. Each step returns a row set. The rows in this set are either used by the next step or, in the last step, are returned to the application issuing the SQL statement.

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A row source is a row set returned by a step in the execution plan along with a control structure that can iteratively process the rows. The row source can be a table, view, or result of a join or grouping operation.

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SQL Execution

During execution, the SQL engine executes each row source in the tree produced by the row source generator. This is the only mandatory step in DML processing.

During execution, if the data is not in memory, then the database reads the data from disk into memory. The database also takes out any locks and latches necessary to ensure data integrity and logs any changes made during the SQL execution. The final stage of processing a SQL statement is closing the cursor.

If the database is configured to use In-Memory Column Store (IM column store), then the database transparently routes queries to the IM column store when possible, and to disk and the database buffer cache otherwise. A single query can also use the IM column store, disk, and the buffer cache. For example, a query might join two tables, only one of which is cached in the IM column store.

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