Project Management Database and SQL Server Katmai New Features Qingsong Yao

Disclaimer The content in this slides is demonstration only. Please do your own research before trying to apply either Sparse or Compression technology to your product server.

Topics Katmai Related New Features A Project Management System Experimental result Reference

Sparse Columns Sparse is purely a storage attribute DDL support for specifying a column as “Sparse” – CREATE TABLE t (id int, sparseProp1 int SPARSE); – ALTER TABLE t ALTER COLUMNS sparseProp1 DROP SPARSE; No Query/DML behavior changes for a sparse column Column metadata will have a bit to indicate the sparse attribute Storage Optimization: – Sparse vector design: 0 bytes stored for a NULL value – Have overhead for not null values (4 byte per not null column + 4 byte header)

Sparse Columns Restrictions Sparse columns cannot be part of a key in clustered index or a PK index or part of a partition key. Sparse column cannot be the key column for an unique index. Unique constraints are also not allowed on sparse columns Sparse columns cannot be defined as "non-null" and cannot have any "default" values. Rules" are not supported on sparse columns.

Filtered Indices “Filtered index” is such a mechanism that allows the table designer to define a regular index that optionally includes a simple filter (predicate) on that table to specify the qualifying rows that need to be indexed for that index. Examples: – Create Index filtered_index on WSS.List(Author) where ListId = 5 Examples of valid filter expressions include – Listid = 10 and folderid > 20 – Listid = 10 and folderid > 20 and folderid < 50 – Listid in (10, 20, 30) – Listid in (10, 20) and folderid in (15, 25)

Filtered Indices TECHNOLOGYIndex Maintenance Cost Filtering Capability Large number of indices Regular IndexHigh for IO; Low for CPU NoneNo (due to high maintenance costs) Index ViewsLow for IO; High for CPU Allows Complex filtering No (due to high maintenance costs) Filtered IndexLow for IO; Low for CPU Simple filtersYes (due to lowered maintenance costs) Support Online Operation, Alter Index, Partition Table, Index hints, and DTA Has side impact on query parameterization (because we have to do predicate matching). Sparse Column and Not-null Filtered Indices like: Create index on t1(c1) where c1 is not null are very helpful on querying/storing “sparse” columns (no impact on query parameterization).

Data Compression Different Compression Types – Vardecimal Compression (SQL Server 2005 SP2) – Row compression Row compression contains vardecimal compression – Page compression Page compression contains row compression Main Focus was data warehouse scenarios – But very useful for certain OLTP scenarios as well Main goal: Enabling compression does not require application changes Compression only supported in Enterprise Edition Microsoft Confidential8

Row compression Light-weight compression – Useful for certain OLTP scenarios All columns stored as variable data in new record format Reduce overhead per column (4-bits vs 2 bytes) Store minimal number of bytes per value: – Leading zero bytes removed for int, smallint, tinyint, bigint, datetime, smalldatetime, money, smallmoney, real, float – Trailing spaces removed for char, nchar, binary – Decimal / Numeric vardecimal compressed (same compression as vardecimal compression) NULL / 0 value take no space (besides overhead) No compression for varchar, nvarchar, varbinary, text, ntext, image 9Microsoft Confidential

Page Compression Compress all data on a single data page Compress ‘similar’ column values by only storing the value once on the page instead of multiple times Two page compression algorithms – Column Prefix – Dictionary User cannot choose algorithm, both algorithms are always applied Page Compression includes row compression 10Microsoft Confidential

Topics Katmai Related New Features A Project Management System Experimental result Reference

Data Sources Three tables store main workitem information Table has 17 regular columns (have meaningful name, and always have not null values), and all other columns has predefined random name, and random data type Views are defined on these columns to assign meaningful names to the columns No. Columns No. RowsNo. PagesMin Rec Size Max Rec Size Avg. Rec Size WorkItems Were WorkItems Latest WorkItemAre: has identity had timestamp columns

Column Distribution Information Column data type distribution: Null value distribution Data TypesCount int31 datetime40 float63 binary1 nvarchar(256)226 =100% null >99% null>95% null>80% null>60% null WorkItemsWere WorkItemsLatest

Topics Katmai Related New Features SQL Project Management database overview Experimental result Reference

Summary Using WorkItemsWere as source table, try following cases: – Compress table using page compression ALTER TABLE WorkItemsWere REBUILD PARTITION = ALL WITH (DATA_COMPRESSION = PAGE) – Compress table using row compression ALTER TABLE WorkItemsWere REBUILD PARTITION = ALL WITH (DATA_COMPRESSION = ROW) – Find all columns have more than 67% null, and change to sparse 1.insert into temp table select * into WorkItemsWere_Temp from WorkItemsWere 2.truncate data truncate table WorkItemsWere 3.change sparse script alter table WorkItemsWere alter column [Fld10004] add sparse 4.insert data back insert into WorkItemsWere select * from WorkItemsWere_temp 5.rebuild index ALTER INDEX [PK_WorkItemsWere] ON [dbo].[WorkItemsWere] REBUILD

Result No. Pages Min Rec Size Max Rec Size Avg. Rec Size Estimated Saving Time (mins) Before Sparse (31.5%) (36%) 20 Page Compressi on (16%) (15%) (27%) 3 Row Compressi on (36%) (46%) 3 Heap Rebuild (81%) Space Saving: Page Compress > Sparse > Row Compress > normal Procedure sp_estimate_data_compression_savings can estimate space saving without doing the actual compression (but it is not very accurate)

CPU Overhead The database server have 16G memory, while table WorkItemsWere (the largest table) is 4G which mean all data can be in the cache, and physical I/Os are likely be 0. Sparse and Compression can save Logical I/O since they require reading less pages. Sparse and Compression can increase CPU time since the data need to be uncompressed.

Table Scan CPU Time Result Cold Run (ms)Warm Run (ms)Logical I/OPhysical I/O Normal Rebuild (97%) Sparse (108%) Page Compression (178%) Row Compression (136%) Select 17 regular columns + 27 predefined columns (with at least 20% not null value) + 10 random predefined columns Result shows that sparse case has less CPU Overhead. Next slide shows the reason.

Table Scan CPU Time Result (2) Select 17 Regular columns select 10 random sparse columns Cold Run (ms) Warm Run (ms) Cold Run (ms) Warm Run (ms) Normal Sparse (89%) (101%) Page Compression (187%) (85%) Row Compression (149%) (87.5%) Sparse does not have negative impact on regular columns. Extracting Null values from sparse columns has higher CPU overhead than page compression and row compression case. Page Compression and Row Compression are in table level, the CPU overhead of uncompressing Not Null values are higher.