1 query processing
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The document discusses the process of query processing, which includes translating SQL queries into relational algebra, generating an evaluation plan, and executing the query. It provides examples of translating a SQL query into relational algebra expressions and generating an annotated evaluation plan specifying the detailed strategy for evaluating the query.
1 query processing
- 1. 1. Query Processing 2. Translating SQL queries into RA 3. Evaluation Plan 4. Query Execution 5. Query Optimization 6. Translation Rules 7. Cost Estimations
- 2. 1. Query Processing 1. Query Processing ▪ Aim of query processing - to find information in one or more databases, - and deliver it to the user quickly and efficiently. - to choose the most cost effective. ▪ Translation of queries - into expressions that can be used at physical level of file system. - Includes query optimization and query evaluation.
- 3. 1. Query Processing 1. Query Processing ▪ Typical steps when processing a high-level query (e.g. SQL query) Query tree internal representation of the query Execution strategy how to retrieve results of query
- 4. 2. Translating SQL queries into RA 2. Translating SQL queries into RA ▪ Translate query into its internal form. - This is then translated into Relational Algebra(RA). - The parser checks syntax, verifies relations. ▪ A RA expression may have many equivalent expressions. ▪ Example Σbalance<2500(πbalance(account)) Is equivalent to Πbalance(σbalance<2500(account)) Each relational algebra operation can be evaluated using one of several different algorithms. Correspondingly, a relational-algebra expression can be evaluated in many ways.
- 5. 3. Evaluation Plan 3. Evaluation Plan ▪ Annotated expression specifying detailed evaluation strategy. ▪ Example Use an index on balance to find accounts with balance < 2500, Or perform complete relation scan and discard accounts with balance ≥ 2500. Initial canonical query tree Book (access#, title) Member (ticket#, name) Loan(loanedbook,loanedto) Select member.name rom book, loan, member where book.title = "dracula" and member.ticket# = loan.loanedto and loan.loanedbook = book.access#
- 6. 4. Query Execution 4. Query Execution For each operation (join, select, project, aggregation …) - Typical algorithms (e.g. Binary search for simple selection) - Specific or not to storage structure and access paths Book (access#, title) Member (ticket#, name) Loan(loanedbook,loanedto) Select member.name From book, loan, member where book.title = "dracula" and member.ticket# = loan.loanedto and loan.loanedbook = book.access#
- 7. 4. Query Execution 4. Query Execution