Optimizing recursive queries in sql

BY: Mark Gruszecki

What is a Recursive Query? Definition(s) and Algorithm(s) Optimization Techniques Practical Issues Impact of each Optimization Conclusion

CREATE RECURSIVE VIEW R(d, i, j, v) AS (SELECT 1, i, j, v FROM TUNION ALLSELECT d+1, R.i, T.j, R.v + T.vFROM R JOIN T ON R.j = T.iWHERE d < 8);

Any kind of Hierarchical Data is more easily accessed using a Recursive Query.

For example, displaying Employees in an orginizational chart.

A base table T is defined as T(i, j, v) with primary key (i, j) and v representing a numerical value.

Define the result table from a recursive query by R(d, i, j, v) with primary key (d, i, j).

The queries of interest are of the form R[k] = T T T … T where is the recursive query.

Let G = (V, E) be a directed graph with n vertices and m edges.

The algorithm to evaluate optimizations is: R = k

kR ][

1. Early evaluation of row select conditions.

2. Deleting Duplicate Rows.3. Indexing base and result tables for

efficient joins.

Used on Queries of the form:SELECT i , j, v FROM RWHERE <condition>

Two Cases: If the “WHERE” condition doesn’t participate

in the JOIN condition of the recursion. The “WHERE” condition is part of the JOIN in

the recursion.

Suppose we want to know which vertices of a graph are connected to a specific node (e.g. all dependants of a particular person).

What if G is complete, dense, sparse, or even a tree?

Queries are optimized by deleting duplicate rows at each step.

Two Choices:

Index based on i and j. Gives rise to an optimal Hash Join Cannot uniquely identify rows

Index based on primary key of T and R Can uniquely identify rows

The possible size of a result table of a recursive query is |T| x |T| x … |T|.

In general, if there are n recursive steps and T has m entries, then there are m^n.

Early Evaluation of Row Selection Conditions Good in all cases

Deleting Duplicate Rows Good in certain cases, but not necessary in all

Indexing Base and Result Tables for Efficient Joins If G is highly connected, slight loss of

performance (Hash Collisions). Otherwise, this is the best optimization.

Recursive Queries are an important tool to be exposed to when dealing with databases.

There are three types of optimizations of these queries discussed: Early evaluation of row selections Deleting duplicate rows Indexing for more efficient joins

Every optimization is good, but some are more applicable in certain circumstances than others.

Carlos Ordonez (2005) Optimizing Recursive Queries in SQL. Retrieved February 10, 2009 from IEEE Computer Science Digital Library

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