Knoldus Inc

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Build a highly scalable cost optimization service

Impact :

About the organization

SQream provides an analytics platform that minimizes Total Time to Insight (TTTI) for time-sensitive data, on-prem and on-the-cloud. Designed for tera-to-petascale data, the GPU-powered platform enables enterprises to rapidly ingest and analyze their growing data – providing full-picture visibility for improved customer experience, operational efficiency, and previously unobtainable business insights.

Challenges

Approach

Sqream DB has a Rule-based query optimizer, but it is impossible to correctly optimize the query without considering the data, data skewness, workloads, and the variety of data. The rule-based optimizer applies different rules to optimize the query, so to cover all the plans, more rules are needed, which results in many complex rules as well as we have to add hints to make those rules work. Still, the required query efficiency can not be achieved for complex queries. Hence, the idea of Cost-Based Optimization looks lucrative, but it is a hard problem to solve in distributed databases. Since the need was urgent, we narrowed it down to Calcite which has a lot of inbuilt rules, as well as a cost-based planner known as Volcano. We wanted to leverage as much functionality from Volcano to enable us to build a Cost-Based Optimizer for Sqream DB. Calcite offers you a framework to build your own rules as well the framework is highly customizable to support all the needs that we have.

The Solution

The primary goals for us were to

To achieve these goals the following High-Level Architecture was proposed to build a highly scalable cost optimization service as described in the diagram below.

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01
Parsing

The whole process starts with parsing. A query, to be understood by the database engine, must first be parsed using an SQL parser, which takes a string of characters and tries to deduce its syntactic structure in the form of a parse tree. It uses a set of syntax rules called language grammar, which defines how an SQL query must look to be considered valid and acceptable to the query engine.

For instance, a rule for parsing SQL SELECT statements might look like this:
select:

<SELECT> expressionList
[<FROM> table]
[<WHERE> condition]
[<GROUP> <BY> groupingList]
[<HAVING> condition]

It declares that a SELECT statement must start with the keyword SELECT, followed by a list of fields and/or expressions to select (could be only one), and optionally any, or all, of the following: a FROM clause specifying the source of data from which to do a select (i.e. a table or a subquery); a WHERE clause filtering selected rows based on a Boolean condition; a GROUP BY clause aggregating rows together based on some keys; and a HAVING clause filtering out some groups if a condition isn't satisfied.

02
Validation

The purpose of validation is to test the semantic correctness of the query, i.e. whether a query written by a user makes sense or not. Validation ensures every object mentioned in the query exists and every operation specified can be done. If it isn't so, the process halts with an explanatory error message.

Another important purpose of validation is to correctly identify to which exact object (field, table, function) any identifier named in the query really refers and also to assign correct data types to every field, row, or expression that therein occurs.

So how we do the validations:

03
Relational algebra

Relational algebra deals with abstract transformations over sets of data, such as.

Conversion to a Relational Tree:

AST(abstract syntax tree) is not convenient for query optimization because the semantics of its nodes are too complicated. It is much more convenient to perform query optimization on a tree of relational operators, defined by the RelNode subclasses, such as Scan, Project, Filter, Join, etc. We use SqlToRelConverter, another monstrous class of Apache Calcite, to convert the original AST into a relational tree.

04
Query optimization

Query optimization is a process in which the original query is transformed according to a set of rules into some other, equivalent query that is faster to run and/or requires less computational resources for its execution.

Optimization is a process of conversion of a relation tree to another relational tree. You may do rule-based optimization with heuristic or cost-based planners, HepPlanner and VolcanoPlanner respectively. You may also do any manual rewrite of the tree without a rule. Apache Calcite comes with several powerful rewriting tools, such as RelDecorrelator and RelFieldTrimme.

Result

The result was a highly secured, performant, extensible compiler frontend service that successfully optimized all the queries and generate the corresponding physical tree.

Client Testimonial

“ I want to thank Knoldus for their great contribution in building the CBO for Sqream enabling us to successfully integrate calcite into our system.” -Gill Cohen (Sqream Project Manager)

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