query-planner-guide

Overview

The Query Planner Guide offers comprehensive insights into building your own query planner, a crucial component of any database management system (DBMS). A query planner's role is to generate an efficient plan that outlines the steps required for executing database queries, thus ensuring rapid data retrieval for users.

By navigating through this guide, you will learn the intricacies of implementing a cost-based query planner with practical examples, making it accessible even for those unenthused by mathematical complexities.

Who is This Guide For?

This guide is ideal for:

• Individuals who have previously worked with query engines.
• Curious minds eager to create their own database solutions.
• Anyone looking to deepen their database knowledge without the extensive math background.

Learning Goals

By following this guide, you will:

• Grasp the essential concepts of query planning.
• Acquire the skills to write your own query planner.

Understanding Query Engine Architecture

A typical query engine architecture includes the following components:

• Query Parser: Parses the user’s input, typically in a human-readable format (like SQL).
• Query Planner: Generates the optimal execution plan for the query.
• Query Executor: Executes the plan devised by the planner.

graph TD
    user((user))
    parser[Query Parser]
    planner[Query Planner]
    executor[Query Processor]
    user -- text query --> parser
    parser -- AST --> planner
    planner -- physical plan --> executor

Types of Query Planners

Query planners fall into two main categories:

• Heuristic Planner: Utilizes predefined rules to create a query plan.
• Cost-Based Planner: Determines the best plan based on the cost associated with executing the query.

Delving into Cost-Based Query Planners

Cost-based query planners generally consist of two main phases:

• Plan Enumeration: Identifies all possible equivalent plans for a given query.
• Query Optimization: Chooses the most efficient plan from the list, typically the one with the lowest cost, defined by a specific cost model.

The exploration phase can be enhanced using various tree search algorithms, such as:

• Exhaustive search (deterministic dynamic programming)
• Randomized search (randomized tree search)

Key Concepts in Query Planning

• Logical Plan: A data structure representing the steps required to execute a query.
• Physical Plan: Contains the implementation details for each logical plan, allowing for various execution strategies (e.g., JOIN methods like HASH JOIN, MERGE JOIN).
• Equivalent Group: A set of logical expressions that yield the same results.
• Transformation Rule: Rules that help transform one logical plan into another equivalent logical plan based on specific properties.

Building Your Cost-Based Query Planner

The implementation of a query planner typically follows a structured approach:

1. Exploration/Search Phase: Generate multiple plans using transformation rules.
2. Implementation/Optimization Phase: Optimize the generated plans to identify the best option.

Example of Logical Plan Generation

Here’s a brief illustration of how a logical plan might be structured:

graph TD
    1["PROJECT tbl1.id, tbl1.field1, tbl2.field1, tbl2.field2, tbl3.id, tbl3.field2, tbl3.field2"];
    2["JOIN"];
    3["SCAN tbl1"];
    4["JOIN"];
    5["SCAN tbl2"];
    6["SCAN tbl3"];
    1 --> 2;
    2 --> 3;
    2 --> 4;
    4 --> 5;
    4 --> 6;

In summary, this guide serves as a practical resource for anyone interested in developing their own query planner. It walks you through essential theory, practical examples, and hands-on coding to help you understand and implement a functional cost-based query planner effectively.