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Tuesday, May 27, 2014

SQL Server Storage Enigne: SELECT Statement Life Cycle Summary

Figure shows the whole life cycle of a SELECT query, described here:

1. The SQL Server Network Interface (SNI) on the client established a connection to the SNI on the SQL Server using a network protocol such as TCP/IP. It then created a connection to a TDS endpoint over the TCP/IP connection and sent the SELECT statement to SQL Server as a TDS (Tabular data stream) message.

2. The SNI on the SQL Server unpacked the TDS message, read the SELECT statement, and passed a “SQL Command” to the Command Parser.

3. The Command Parser checked the plan cache in the buffer pool for an existing, usable query plan that matched the statement received. When it didn’t find one, it created a query tree based on the SELECT statement and passed it to the Optimizer to generate a query plan.

4. The Optimizer generated a “zero cost” or “trivial” plan in the pre-optimization phase because the statement was so simple. The query plan created was then passed to the Query Executor for execution.
    Next steps involve in making execution plan will be

         Phase 0 — During this phase the optimizer looks at nested loop joins and won’t consider            parallel operators.The optimizer will stop here if the cost of the plan it has found is < 0.2. A plan generated at this phase is known as a "transaction processing", or TP, plan.
         Phase 1 — it uses subset of the possible optimization rules and looks for common patterns for which it already has a plan. The optimizer will stop here if the cost of the plan it has found is < 1.0. Plans generated in this phase are called "quick" plans.
         Phase 2 — This final phase is where the optimizer pulls out all the stops and is able to use all of its optimization rules. It also looks at parallelism and indexed views (if you’re running Enterprise Edition). Plans created in this phase have an optimization level of “Full.”

5. At execution time, the Query Executor determined that data needed to be read to complete the query plan so it passed the request to the Access Methods in the Storage Engine via an OLE DB interface.

6. The Access Methods needed to read a page from the database to complete the request from the Query Executor and asked the Buffer Manager to provision the data page.

7. The Buffer Manager checked the data cache to see if it already had the page in cache. It wasn’t in cache so it pulled the page from disk, put it in cache, and passed it back to the Access Methods.

8. Finally, the Access Methods passed the result set back to the Relational Engine to send to the client.

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