Troubleshooting SQL Server Spatial Query Performance: - PowerPoint Presentation

502 views
Uploaded On 2016-05-06

Troubleshooting SQL Server Spatial Query Performance: - PPT Presentation

A Deep Dive into Spatial Indexing Michael Rys Principal Program Manager Microsoft Corporation SQLServerMike DBI405 Q Why is my Query so Slow A Usually because the index isnt being used ID: 306948

spatial index filter query index spatial query filter sql server object table number cell grid window microsoft cells 2012

Link:

Copy

Embed:

<iframe width="560" height="315" src="https://www.docslides.com/embed/306948" frameborder="0" allowfullscreen></iframe>

Download Presentation from below link

Download Presentation The PPT/PDF document "Troubleshooting SQL Server Spatial Query..." is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.

Presentation Transcript

Slide1

Troubleshooting SQL Server Spatial Query Performance: A Deep Dive into Spatial Indexing

Michael RysPrincipal Program ManagerMicrosoft Corporation@SQLServerMike

DBI405Slide2

Q: Why is my Query so Slow?

A: Usually because the index isn’t being used.Q: How do I tell?

A: SELECT * FROM T WHERE g.STIntersects(@x) = 1

NO INDEX

INDEX!Slide3

Hinting the Index

Spatial indexes can be forced if needed.SELECT * FROM T

WHERE g.STIntersects(@x) = 1Use SQL Server 2008 SP1 or later!

WITH(INDEX(

T_g_idx

))Slide4

But Why Isn't My Index Used?

Plan choice is cost-basedQO uses various information, including cardinality

When can we estimate cardinality?Variables: neverLiterals: not for spatial since they are not literals under the covers

Parameters: yes, but cached, so first call matters

DECLARE @x geometry = 'POINT (0 0)'

SELECT *

FROM T

WHERE

T.g.STIntersects(@x) = 1

SELECT *FROM TWHERE T.g.STIntersects('POINT (0 0)') = 1

EXEC sp_executesql N'SELECT * FROM T

WHERE

T.g.STIntersects

(@x) = 1',

N'@x

geometry', N'POINT (0 0)'Slide5

Spatial Indexing Basics

In general, split predicates in twoPrimary filter finds all candidates, possibly

with false positives (but never false negatives)Secondary filter removes false positivesThe index provides our primary filterOriginal predicate is our secondary filterSome tweaks to this scheme

Sometimes possible to skip secondary filter

Primary Filter

(Index lookup)

Secondary Filter

(Original predicate)

ESlide6

Using B+-Trees for Spatial Index

SQL Server has B+-TreesSpatial indexing is usually done through other structuresQuad tree, R-TreeChallenge: How do we repurpose the B+-Tree to handle spatial queries?

Add a level of indirection!Slide7

Mapping to the B+-Tree

B+-Trees handle linearly ordered sets wellWe need to somehow linearly order 2D space Either the plane or the globeWe want a locality-preserving mapping from the original space to the line

i.e., close objects should be close in the indexCan’t be done, but we can approximate itSlide8

SQL Server Spatial Indexing Story

Requires bounding boxOnly one grid

Planar IndexGeographic Index

No bounding box

Two top-level projection grids

1. Overlay a grid on the spatial object

2. Identify grids for spatial object to store in index

3. Identify grids for query object(s)

4. Intersecting grids identifies candidates

Indexing Phase

Primary Filter

Secondary Filter

5. Apply actual CLR method on candidates to find matchesSlide9

SQL Server Spatial Indexing Story

Multi-Level GridMuch more flexible than a simple grid

Hilbert numberingModified adaptable QuadTreeGrid index features4 levelsCustomizable grid subdivisionsCustomizable maximum number of cells per object (default 16)

NEW IN SQL Server 2012:

New Default tessellation with 8 levels of cell nestingSlide10

Multi-Level Grid

Deepest-cell Optimization: Only keep the lowest level cell in index

Covering Optimization: Only record higher level cells when all lower

cells are completely covered by the object

(“cell 0”)

Cell-per-object Optimization: User restricts max number of cells per object

/4/2/3/1Slide11

Implementation of the Index

Persist a table-valued functionInternally rewrite queries to use the table

Prim_keygeography1

Prim_key

cell_id

sridcell_attr10x00007

0x00007

0x0000A

0x0000B

0x0000C

10x0000D42

020x00014421

Base Table T

Internal Table for sixdCREATE SPATIAL INDEX sixd

ON T(geography)0 – cell at least touches the object (but not 1 or 2)

1 – guarantee that object partially covers cell2 – object covers cell

15 columns and 895 byte limitation

Spatial Reference ID

Have to be the same to produce match

Varbinary

(5) encoding of grid cell idSlide12

Auto Grid Spatial Index

New spatial index Tessellations: geometry_auto_gridgeography_auto_gridUses 8 Grid levels instead of the previous 4No GRIDS parameter needed (or available)Fixed at HLLLLLLL

default number of cells per object:8 for geometry 12 for geographyMore stable performance for windows of different sizefor data with different spatial density

For default values:

Up to 2x faster for longer queries > 500

More efficient primary filter Fewer rows returned10ms slower for very fast queries < 50 msIncreased tessellation time which is constantSlide13

Spatial Index Performance

New grid gives much stable performance for query windows of different size

Better grid coverage gives fewer high peaksSlide14

Index Creation and Maintenance

Create index example GEOMETRY:CREATE SPATIAL INDEX sixd ON

spatial_table(geom_column)WITH (BOUNDING_BOX = (0, 0, 500, 500), GRIDS = (LOW, LOW, MEDIUM, HIGH),

CELLS_PER_OBJECT

= 20)

Create index example GEOGRAPHY:

CREATE SPATIAL INDEX

sixd ON spatial_table

(geogr_column)USING GEOGRAPHY_GRID

WITH (GRIDS = (LOW, LOW, MEDIUM, HIGH), CELLS_PER_OBJECT = 20)NEW IN SQL Server 2012

(equivalent to default creation):CREATE SPATIAL INDEX sixd ON spatial_table(geom_column)

USING GEOGRAPHY_AUTO_GRID

WITH (CELLS_PER_OBJECT = 20)

Use ALTER and DROP INDEX for maintenance.Slide15

demo Indexing and PerformanceSlide16

Spatial queries supported by index in SQL Server

Geometry:STIntersects() = 1

STOverlaps() = 1STEquals()= 1STTouches() = 1STWithin

() = 1

STContains

() = 1

STDistance

() < val

STDistance() <= val

Nearest NeighborFilter() = 1Geography

STIntersects() = 1 STOverlaps() = 1

STEquals

()= 1

STWithin

() = 1

STContains

() = 1

STDistance

() <

val

STDistance

() <=

val

Nearest Neighbor

Filter() = 1

New in SQL Server 2012Slide17

How Costing is Done

The stats on the index contain a trie constructed on the string form of the packed binary(5) typed Cell ID.

When a window query is compiled with a sniffable window object, the tessellation function on the window object is run at compile time. The results are used to construct a trie for use during compilation. May lead to wrong compilation for later objectsNo costing on:Local variables, constants, results of expressions

Use different indices and different stored

procs

to account for different query characteristicsSlide18

Understanding the Index Query PlanSlide19

Seeking into a Spatial Index

Minimize I/O and random I/OIntuition: small windows should touch small portions of the indexA cell 7.2.4 matches Itself

AncestorsDescendants

Spatial Index S

7.2

7.2.4Slide20

Understanding the Index Query Plan

T(@g)

Spatial Index Seek

Ranges

Remove dup ranges

Optional SortSlide21

Spatial index tessellation

Better and more continuous coverage

64 cells 128 cells 256 cells

Fully contained cells

Partially contained cellsSlide22

Query window number of cells

Typical spatial query performance

Optimal value (theoretical) is somewhere between two extremes

Time needed to

process false positives

Default values:

512 - Geometry AUTO grid

768 - Geography AUTO grid

1024 - MANUAL grids

SELECT * FROM table t WITH (

SPATIAL_WINDOW_MAX_CELLS

=256)

WHERE

t.geom.STIntersects

(@window)=1;Slide23

Query Window Hinting (SQL Server 2012)

SELECT * FROM table t with(SPATIAL_WINDOW_MAX_CELLS=1024)WHERE t.geom.STIntersects

(@window)=1Used if an index is chosen (does not force an index)Overwrites the default (512 for geometry, 768 for geography)Rule of thumb:Higher value makes primary filter phase longer but reduces work in secondary filter phase

Set higher for dense spatial data

Set lower for sparse spatial data Slide24

Index Hinting

FROM T WITH (INDEX (<Spatial_idxname>))Spatial index is treated the same way a non-clustered index is

the order of the hint is reflected in the order of the indexes in the planmultiple index hints are concatenatedno duplicates are allowedThe following restrictions exist:The spatial index must be either first in the first index hint or last in the last index hint for a given table.

Only one spatial index can be specified in any index hint for a given table.Slide25

demo Spatial

Query HintingSlide26

Additional Query Processing

SupportIndex intersection

Enables efficient mixing of spatial and non-spatial predicatesMatchingNew in SQL Server 2012: Nearest Neighbor queryDistance queries: convert to STIntersectsCommutativity: a.STIntersects(b) =

b.STIntersects

(a)

Dual:

a.STContains

(b) = b.STWithin(a)Multiple spatial indexes on the same columnVarious bounding boxes, granularitiesOuter references as window objectsEnables spatial join to use one indexSlide27

Other Spatial Performance Improvements in SQL Server 2012

Spatial index build time for point data can be as much as four to five times faster Optimized spatial query plan for STDistance

and STIntersects like queriesFaster point data queriesOptimized STBuffer, lower memory footprintSlide28

Spatial Nearest NeighborMain scenario

Give me the closest 5 Italian restaurantsExecution plan SQL Server 2008/2008 R2: table scan

SQL Server 2012: uses spatial indexSpecific query pattern requiredSELECT

TOP

(5) *

FROM Restaurants r

WHERE

r.type

= ‘Italian’

AND r.pos.

STDistance(@me) IS NOT NULLORDER BY

r.pos.

STDistance

(@me)Slide29

demo Spatial Performance in SQL Server

2012Slide30

Nearest Neighbor Performance

NN query

vs best current workaround (sort all points in 10km radius)

*Average time for NN query is ~236ms

Find the closest 50 business points to a specific location (out of 22 million in total)Slide31

Limitations of Spatial Plan Selection

Off whenever window object is not a parameter:Spatial join (window is an outer reference)Local variable, string constant, or complex expressionHas the classic SQL Server parameter-sensitivity problem

SQL compiles once for one parameter value and reuses the plan for all parameter valuesDifferent plans for different sizes of window require application logic to bucketize the windowsSlide32

Error 8635: Cannot find a plan

Error: The query processor could not produce a query plan for a query with a spatial index hint. Reason: %S_MSG. Try removing the index hints or removing SET FORCEPLAN.Possible Reasons (%S_MSG):The spatial index is disabled or offline

The spatial object is not defined in the scope of the predicateSpatial indexes do not support the comparand supplied in the predicateSpatial indexes do not support the comparator supplied in the predicateSpatial indexes do not support the method name supplied in the predicateThe comparand references a column that is defined below the predicate

The

comparand

in the comparison predicate is not deterministic

The spatial parameter references a column that is defined below the predicate

Could not find required binary spatial method in a conditionCould not find required comparison predicateSlide33

Index Support

Can be built in parallelCan be hinted

File groups/PartitioningAligned to base table or Separate file groupFull rebuild onlyNew catalog views, DDL EventsDBCC Checks

Supportability stored procedures

New in SQL Server 2012:

Index Page and Row Compression

Ca. 50% smaller indices, 0-15% slower queries

Not supported

Online rebuild

Database Tuning advisorSlide34

SET Options

Spatial indexes requires:ANSI_NULLS: ONANSI_PADDING: ONANSI_WARNINGS: ONCONCAT_NULL_YIELDS_NULL: ON

NUMERIC_ROUNDABORT: OFFQUOTED_IDENTIFIER: ONSlide35

Spatial Indices and Partitions and Filegroups

Default partitioned to the same filegroups as the base table. Overwrite with: [ ON {

filegroup_name | "default" } ]If filegroup_name is specified, the index will be placed on the specified filegroup regardless of the table’s partitioning scheme. If “default” is specified, the base table’s default filegroup/partitioning scheme is applied.Altering the base table’s partition scheme is not allowed unless the spatial index was created with the “ON

filegroup

” option (and is hence not aligned with the partitioning anyway). The index has to be dropped and then the base table repartitioned. Slide36

Spatial Catalog Views

sys.spatial_indexes catalog viewsys.spatial_index_tessellations catalog view

Entries in sys.indexes for a spatial index:A clustered index on the internal table of the spatial indexA spatial index (type = 4) for spatial indexAn entry in sys.internal_tables

An entry to

sys.index_columns

Slide37

sp_spatial_help_geometry_histogram

sp_spatial_help_geography_histogram

Used for spatial data and index analysis

New Spatial Histogram Helpers

Histogram of 22 million business points over US

Left: SSMS view of a histogram

Right: Custom drawing on top of Bing MapsSlide38

Indexing Support Procedures

sys.sp_help_spatial_geometry_indexsys.sp_help_spatial_geometry_index_xml

sys.sp_help_spatial_geography_indexsys.sp_help_spatial_geography_index_xmlProvide information about index:

64 properties

10 of which are considered coreSlide39

sys.sp_help_spatial_geometry_index

Arguments

Results

in property name/value pair table of the format:

Parameter

Type

Description

tabname

nvarchar

(776)

the name of the table for which the index has been specified

indexname

sysname

the index name to be investigated

verboseoutput

tinyint

0 core set of properties is reported

1 all properties are being reported

query_sample

geometry

A representative query sample that will be used to test the usefulness of the index. It may be a representative object or a query window.

PropName

nvarchar

(256)

PropValue

sql_variantSlide40

sys.sp_help_spatial_geography_index_xml

Arguments

ParameterType

Description

tabname

nvarchar

(776)

the name of the table for which the index has been specified

indexnamesysname

the index name to be investigated

verboseoutput

tinyint

0 core set of properties is reported

1 all properties are being reported

query_sample

geography

A representative query sample that will be used to test the usefulness of the index. It may be a representative object or a query window.

xml_output

xml

This is an

output

parameter that contains the returned properties in an XML fragmentSlide41

Some of the returned Properties

Property

Type

Description

Base_Table_Rows

Bigint

All

Number of rows in the base table

Index properties

All

index properties: bounding

box, grid densities, cell

per object

Total_Primary_Index_Rows

Bigint

All

Number of rows in the index

Total_Primary_Index_Pages

Bigint

All

Number of pages in the index

Total_Number_Of_ObjectCells_In_Level0_For_QuerySample

Bigint

Core

Indicates whether the representative query sample falls outside of the bounding box of the geometry index and into the root cell (level 0 cell). This is either 0 (not in level 0 cell) or 1. If it is in the level 0 cell, then the investigated index is not an appropriate index for the query sample.

Total_Number_Of_ObjectCells_In_Level0_In_Index

Bigint

Core

Number of cell instances of indexed objects that are tessellated in level

For

geometry indexes, this will happen if the bounding box of the index is smaller than the data domain. A

high number of objects in level 0 may require a costly application of secondary filters if the query window falls partially outside the bounding box.

If the query window falls inside the bounding box, having a high number of objects in level 0 may actually improve the performance. Slide42

Some of the returned Properties

Property

Type

Description

Number_Of_Rows_Selected_By_Primary_Filter

bigint

Core

P = Number

of rows selected by the primary filter.

Number_Of_Rows_Selected_By_Internal_Filter

bigint

Core

S = Number

of rows selected by the internal filter. For these rows, the secondary filter is not called.

Number_Of_Times_Secondary_Filter_Is_Called

bigint

Core

Number of times the secondary filter is called.

Percentage_Of_Rows_NotSelected_By_Primary_Filter

float

Core

Suppose there are N rows in the base table, suppose P are selected by the primary filter. This is (N-P)/N as percentage.

Percentage_Of_Primary_Filter_Rows_Selected_By_Internal_Filter

float

Core

This

is S/P as a percentage. The higher the percentage, the better is the index in avoiding the more expensive secondary filter.

Number_Of_Rows_Output

bigint

Core

O=Number

of rows output by the query.

Internal_Filter_Efficiency

floatCoreThis is S/O as a percentage.

Primary_Filter_Efficiencyfloat

CoreThis is O/P as a percentage. The higher the efficiency is, the less

false positives have to be processed by the secondary filter.Slide43

Spatial Tips on index settings

Some best practice recommendations (YMMV):Start out with new default tesselation

Point data: always use HIGH for all 4 level. CELL_PER_OBJECT are not relevant in the case.Simple, relatively consistent polygons: set all levels to LOW or MEDIUM, MEDIUM, LOW, LOW Very complex LineString or Polygon instances:High number of CELL_PER_OBJECT (often 8192 is best)

Setting all 4 levels to HIGH may be beneficial

Polygons or line strings which have highly variable sizes: experimentation is needed.

Rule of thumb for GEOGRAPHY: if MMMM is not working, try HHMM Slide44

What to do if my Spatial Query is slow?

Make sure you are running SQL Server 2008 SP1, 2008 R2 or 2012

Check query plan for use of indexMake sure it is a supported operationHint the index (and/or a different join type)Do not use a spatial index when there is a highly selective non-spatial predicate

Run above index support procedure:

Assess effectiveness of primary filter (

Primary_Filter_Efficiency

)

Assess effectiveness of internal filter (Internal_Filter_Efficiency)

Redefine or define a new index with better characteristicsMore appropriate bounding box for GEOMETRYBetter grid densitiesSlide45

Summary: Spatial Index Improvements in SQL Server 2012

Auto Grid Spatial IndexSpatial Index HintMore supported OperationsSpatial Index Compression

Improved “Create Spatial Index” Time For Point DataSlide46