SQLXPress Visual Query Tuner

Copyright © 2014 Merlon Software Corporation

Agenda

• Introduction to Visual Query Tuner (VQT)• How to generate a plan• How to read a plan• Measuring query performance• Influencing plans• Comparing performance• Auto-tuner

Introduction

• VQT is a Windows® application• Connects to SQLXPress server process running on a NonStop

system• Supports both MX and MP queries

MX version 2.3.4 or later MP version 350 or later

Introduction

• VQT uses a NonStop SQL database to store query details Each query has a name and plain language description

• Keeps a history of ”snapshots” for each query• Each snapshot contains:

Query text Plain language description of snapshot Params, defines, and CONTROL statements Execution plan Execution statistics

Introduction

• Why use VQT? Learn how query is executed Get performance characteristics of query Identify problems in a plan Try remedial changes, for example:

• Update statistics• Database changes (create index)• Query changes (predicate re-write)• Use of SQL CONTROL statements (CQD, CQS)

Compare performance after remediation

How to generate a plan

• Use the Query tab to enter query detail

Color-coded syntax Auto-completion Query text will most

likely be pasted from clipboard

• Click Take Snapshot to save query, generate plan, and collect execution statistics

How to generate a plan

• The first Take Snapshot command prompts for a query name• Subsequent Take Snapshot commands for the same query use the

same name Use Save Snapshot As to save under a new name Use New command to start working on a new query

• Each snapshot is identified by the query name and the time the snapshot was taken

How to generate a plan

• Snapshot Management Take snapshot command adds a new snapshot Delete snapshot command deletes current snapshot

• Also has delete snapshots dialog to delete multiple snapshots with one command

Easy to navigate snapshots from toolbar Snapshot retention – automatically delete oldest snapshot when retention

limit exceeded Use any snapshot as staring point for new query “experiment”

How to read a plan

• Plans are displayed in the Execution Plan tab• Plans displayed in three different ways:

Summary – key attributes of each operator Details – plan outline, plus complete details on each operator Plan Diagram – a truly visual plan depiction

How to read a plan

• Plan Summary (MX)• Lists each operator• Key operator attributes• Similar to HP’s VQP

How to read a plan

• Plan Summary (MP)• Lists each operation• Shows index and table

selectivity

How to read a plan

• Plan Details (MX)• Tree view on left

provides plan outline• Complete plan text on

right shows operator details

• Click a node in the outline to show details for selected operator

• Similar to HP’s VQP, but viewing operator details is much easier

How to read a plan

• Plan Details (MP)• Outline shows steps and

operations• Click on node in outline

to show details of step / operation

How to read a plan

• Plan Diagram (MX)• Plan displayed as a tree• Much more intuitive

layout than traditional tree view

• There is a lot of information in the diagram

How to read an MX plan diagram

• Plan fragments are color-coded• Data flows between fragments shown as heavy dotted line• Data flows within a fragment shown as lighter solid lines• Full table / index scan operators have heavy red borders• Operators that run in parallel are “stacked”• Estimated row count displayed above each operator• Operator details displayed in popup window

How to read a plan

• Operator details can be displayed in a popup window

• Move mouse over an operator to display details

• Move mouse away, popup is dismissed

How to read a plan

• Plan Diagram (MP)• Plan displayed as a tree• Both steps and operations

are represented by nodes• Similar to MX diagram

How to read a plan

• Various ways to mitigate problems displaying large plans Scroll and zoom Fit to page Left to right – may be better than top to bottom on wide-screen monitors Simplified MX plan diagrams

How to read a plan

• Simplified Plan Tree (MX)

• Suppress display of: ESP Exchange Split Top Partition Access

• Fewer nodes to display• Diagram visually

indicates fragment boundaries and parallel operators

Measuring query performance

• Take Snapshot command automatically runs query to gather execution statistics

Can be turned off in Options

• Use Run Query command to explicitly run the query without creating a new snapshot

Use to collect multiple samples of execution statistics Avoid cache bias by running query more than once

• Run Query does not display query results Rows are discarded as soon as they are received by the server process

• Execution Statistics tab shows results from Run Query

Measuring query performance

• Execution Statistics• Shows compile time and

execution time• Per-table statistics, like

those shown by MXCI / SQLCI

• Shows min, max, avg values for multiple samples

Influencing plans

• There are various techniques to try and make query run faster Update Statistics Adjust access paths

• For example, add or modify an index Use SQL Control statements

• For example, force join order, join method, parallelism Adjust the query text

• Rewrite the query while still returning the same set of rows

Influencing plans - Update Statistics

• Use View Table Statistics command

• Current statistics are displayed for tables in the query

• From here you can update statistics

Influencing plans – Access Paths

• Use View Access Paths command

• Access paths for tables in query are displayed

• From here you can create or drop indexes

• Click Object Properties to view properties (including DDL)

Influencing plans – SQL Control Statements

• For MX: CONTROL QUERY DEFAULT CONTROL QUERY SHAPE CONTROL TABLE

• For MP: CONTROL QUERY CONTROL EXECUTOR CONTROL TABLE

• Specify CONTROL statements on Query tab• Stored in database as part of query snapshot

Influencing plans example - MX CQD

• Example of CQD• Click Edit to set CQD

values

Influencing plans example - MX CQD

• Edit CQD dialog• Click on link to get popup help

on a CQD attribute• Many attributes have pick-list

of valid values• Values that are Not Specified

will take default value

Influencing plans example - MX CQS

• CQS tab• Enter CQS statement• Click Show to issue

SHOWSHAPE to get current shape which you can then edit

Drag divider up to make more room when editing shape

Influencing plans example – MP

• Control Executor / Query tab

Parallel Plans MDAM Interactive Access

• Join Control tab Specifies join sequence

and join method

• Control Table tab Additional control table

statements

Influencing plans – adjust query text

• Change the query text in a way that helps the optimizer produce a better plan, but returns the same rows as before

• Avoid using columns as function parameters (including CAST) to allow use of an index

• Add additional redundant predicates to avoid full table scans• Don’t aggregate data in the application, use the SQL aggregate

functions• Only select the columns you actually need

Influencing plans – example use of functions

• Suppose ORDERPLACED is a TIMESTAMP column• To query yesterday’s orders:

Instead of:CAST (O.ORDERPLACED AS DATE) = CURRENT_DATE - INTERVAL '1' DAY

Use:O. ORDERPLACED >= CAST(CURRENT_DATE - INTERVAL '1' DAY AS TIMESTAMP) AND O. ORDERPLACED < CAST(CURRENT_DATE AS TIMESTAMP)

• If there is an index on ORDERPLACED this will avoid a full table scan

Influencing plans – example redundant predicates• Instead of:

select * from dept d where d.deptnum in (1000, 3000, 4000)

• Use:select * from dept d where d.deptnum in (1000, 3000, 4000) and d. deptnum >= 1000 and d. deptnum <= 4000

• This will avoid a full table scan• There are more examples of query rewrite in the VQT User

Guide

Comparing performance

• After you make adjustments to a query, you should determine if the performance has improved

Take a new snapshot Compare the performance with previous snapshots

• Use the View Performance Charts command to compare execution statistics

Comparing performance

• Chart shows stats for all snapshots

• Various Data Values: Response Times Records Read / Used Number of Messages Message Bytes

• Show sample: Minimum Maximum Average

Comparing performance

• This is example uses line chart

• Shows records read and records used

The Auto-tuner

• Try other remediation techniques first• Auto-tuner only works on SELECT statements• May not work on some queries

Publish / subscribe Queries using complex views

• Multi-step wizard• Allows user to take snapshot based on recommended query and

SQL control statements

The Auto-tuner

• Goals Get a robust execution plan Get good (but not necessarily the best) performance

• How it works Analyzes query text Determines available access paths Collects metrics on joins and predicates

• Can use queries against tables, or ask the user Generates recommendations:

• May include create / modify index recommendations• Re-written query text• SQL CONTROL statements

The Auto-tuner

• For robustness: Use nested joins

• Scale well with increased table sizes• Lower memory requirements compared to hash and merge joins• Don’t require temporary disk space for hash or sort

• For performance Determine best join order

• Drive query from table with best filter• For the rest of the join order

– Prefer joins to master tables over detail tables– Prefer joins to well filtered tables– Prefer joins with lower join ratios

The Auto-tuner

• For performance: Support driver table filter with an index Support joins to detail tables with index on foreign key Prohibition on hash joins may be relaxed in some circumstances

The Auto-tuner

• Recommendations report• Includes query re-write

and SQL CONTROL statements

The Auto-tuner

• Post analysis actions• Select a recommendation

and click Go to implement