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Arunesh Choudhary
Arunesh Choudhary

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03 - Query Processing

What is Query processing?

In PostgreSQL, the set of operations carried out by the database engine to carry out a SQL query and deliver the appropriate results is referred to as query processing. It involves a number of steps, such as query execution, query optimization, and query parsing.

Query Parsing: The SQL query must first be parsed and its syntax checked. The query's structure is examined by the parser to make sure it complies with the standards of the SQL language. The parser will issue an error and stop processing the query if there are any syntax mistakes.

Query Rewriting: The next phase is query rewriting after the query has been correctly processed. The query is subject to a number of changes by PostgreSQL to make it simpler and more effective. This entails enlarging views, resolving aliases, and simplifying difficult statements.

Query Optimization: PostgreSQL's query optimizer examines the rewritten query, creates an execution plan, and executes the plan. In an effort to reduce the total execution cost, the optimizer compares several methods for accessing the data, joining tables, and applying filters. It employs a variety of methods to calculate the cost of alternative execution strategies while taking data distribution statistics into account.

Execution Plan Generation: PostgreSQL produces an execution plan based on the optimizer's analysis. The query's execution is outlined in the execution plan in great detail, step by step. The sequence in which tables are accessed, join strategies, and any extra actions like sorting or aggregation are all specified.

Query Execution: The actual execution of the query in accordance with the produced execution plan is the last stage. The execution engine receives data from the disk, does any required actions, such as filtering or sorting, and then provides the user or program with the result set.

Closing Remarks

PostgreSQL uses a number of techniques and algorithms to improve query processing, including the use of indexes, caching, and parallel processing. The objective is to efficiently provide query results while reducing resource utilization and response time.

It's important to note that PostgreSQL's query processing is an intricate and flexible process. Administrators can fine-tune the execution plans for certain workloads or queries by using the configuration settings and tuning tools the database offers.

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