{"id":2201,"date":"2014-03-12T11:00:24","date_gmt":"2014-03-12T15:00:24","guid":{"rendered":"http:\/\/sqlity.net\/en\/?p=2201"},"modified":"2014-11-13T13:41:05","modified_gmt":"2014-11-13T18:41:05","slug":"row-header","status":"publish","type":"post","link":"https:\/\/sqlity.net\/en\/2201\/row-header\/","title":{"rendered":"The Row Header (A Storage Wednesday Post)"},"content":{"rendered":"
\n

Introduction<\/h3>\n

\nIn February 2014, I started the Storage Wednesday series. Since then I have written every Wednesday about the way SQL Server stores and accesses data. The list of posts can be found at the end of the first one: The Page<\/a>.\n<\/p>\n

\nIn continuation of the Storage Wednesday series, today I would like to invite you to discover the way a single row is organized within a data page. (The data page was introduced in The Row Offset Array<\/a>.)\n<\/p>\n

Row Header Example<\/h3>\n

\nTo dive into the row header, we need a table with a few columns, some based on a fixed length data type and some based on a variable length data type. I am going to use this table structure:\n<\/p>\n

\n[sql]\nIF OBJECT_ID('dbo.tst') IS NOT NULL DROP TABLE dbo.tst;
\nCREATE TABLE dbo.tst (
\n id INT IDENTITY(1, 1)
\n PRIMARY KEY CLUSTERED,
\n Col2 INT DEFAULT CHECKSUM(NEWID()),
\n Col3 DATETIME2 DEFAULT SYSUTCDATETIME(),
\n Col4 VARCHAR(2200)
\n DEFAULT ( REPLICATE('4', 1 + ABS(CHECKSUM(NEWID()) % 10)) ),
\n Col5 VARCHAR(2200)
\n DEFAULT ( REPLICATE('5', 1 + ABS(CHECKSUM(NEWID()) % 10)) )
\n );
\nINSERT INTO dbo.tst(Col3)SELECT SYSUTCDATETIME() FROM sys.columns;
\n[\/sql]\n<\/div>\n

\nThis SQL batch (re-)creates the dbo.tst<\/span> table and also inserts a few rows into it. The inserted strings in Col4<\/span> and Col5<\/span> have a random length, so you will see different lengths in every row.\n<\/p>\n

\nLet us look at the first page. In my case the first page is page 1:282 and I am going to use DBCC PAGE<\/span> with display type 3:\n<\/p>\n

\n[sql]\nDBCC TRACEON(3604);
\nDBCC PAGE(0,1,282,3);
\n[\/sql]\n<\/div>\n

\nThe output of these statements is too long to include here. However, for each row on the page you will find a section like this:\n<\/p>\n

\n[sourcecode]\nRecord Type = PRIMARY_RECORD Record Attributes = NULL_BITMAP VARIABLE_COLUMNS
\nRecord Size = 42
\nMemory Dump @0x000000006F36A060<\/p>\n

0000000000000000: 30001400 01000000 f32daf6b 6d7579d1 073e380b 0.......\u00f3-\u00afkmuy\u00d1.>8.
\n0000000000000014: 05000002 0027002a 00343434 34343434 34343435 .....'.*.44444444445
\n0000000000000028: 3535 55 <\/p>\n

Slot 0 Column 1 Offset 0x4 Length 4 Length (physical) 4<\/p>\n

id = 1 <\/p>\n

Slot 0 Column 2 Offset 0x8 Length 4 Length (physical) 4<\/p>\n

Col2 = 1806642675 <\/p>\n

Slot 0 Column 3 Offset 0xc Length 8 Length (physical) 8<\/p>\n

Col3 = 2014-03-03 00:55:57.9169133 <\/p>\n

Slot 0 Column 4 Offset 0x1d Length 10 Length (physical) 10<\/p>\n

Col4 = 4444444444 <\/p>\n

Slot 0 Column 5 Offset 0x27 Length 3 Length (physical) 3<\/p>\n

Col5 = 555 <\/p>\n

Slot 0 Offset 0x0 Length 0 Length (physical) 0<\/p>\n

KeyHashValue = (8194443284a0)
\nSlot 1 Offset 0x8a Length 47
\n[\/sourcecode]\n<\/p><\/div>\n

\nThis output separates all the values for easier digestion. However, it does not cover the entire row header. The complete row data looks like this:\n<\/p>\n

\n[sourcecode]\n0000000000000000: 30001400 01000000 f32daf6b 6d7579d1 073e380b 0.......\u00f3-\u00afkmuy\u00d1.>8.
\n0000000000000014: 05000002 0027002a 00343434 34343434 34343435 .....'.*.44444444445
\n0000000000000028: 3535 55
\n[\/sourcecode]\n<\/div>\n

\nThe row header is split into two pieces, separated from each other by the fixed length column values. The first two bytes, 0x3000<\/span> in our case, contain status bits. This value indicates the type of row and whether the row contains variable length data.\n<\/p>\n

\nThe next two bytes, 0x1400<\/span> in the example, is a pointer to the start of the second half of the header. Remember that al vaues are stored n reverse byte order, so this entry actually means 0x14<\/span> or 20. The remaining bytes of the first row in this output (01000000 f32daf6b 6d7579d1 073e380b<\/span>) contain the values for the fixed length columns id<\/span>, Col2<\/span> and Col3<\/span>. The first two columns are of data type INT<\/span> and take four bytes each, Col3<\/span> is a DATETIME2<\/span> column and uses eight bytes.\n<\/p>\n

\nStarting with byte 0x14<\/span> we have two bytes that contain the number of columns in the row (0x0500<\/span>). After that follows the NULL-bitmap. The NULL-bitmap contains one bit for each column in the row. That means the length of the NULL-bitmap is one byte for every eight columns in the row.\n<\/p>\n

\nFollowing the NULL-bitmap, at position 0x17<\/span> in our example is the variable length section of the row. It is completely omitted if the row does not contain variable length columns. It starts with a two-byte field that contains the number of variable lengths columns (0x0200<\/span>). It is followed by as many two-byte pointers, each pointing to the first byte after the respective variable length column. The variable length values follow right after those pointers, starting a position 0x1d<\/span> in the example.\n<\/p>\n

\nIn total, there are six bytes in the header, plus the NULL-bitmap and plus the variable length column offset array that starts out with a two byte count. Every row in SQL Server 2012 has a NULL-bitmap (in some earlier versions that was optional), so the minimum header length is seven bytes if the row consists of less <\/span><=8[\/tt] columns and no column is of a variable length data type.\n<\/p>\n

Summary<\/h3>\n

\nEvery row in SQL Server has its own header, the row header. It contains metadata describing the record and is at least seven bytes in length.\n<\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

The row header contains metadata describing the row. It contains information about the row type, the column count and a NULL-bitmap. Read on to get all the details.<\/p>\n

[more…]<\/a><\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[5,27,14,105],"tags":[111,107,128,15,109],"class_list":["post-2201","post","type-post","status-publish","format-standard","hentry","category-general","category-series","category-sql-server-internals","category-storage-wednesday","tag-dbcc-page","tag-page","tag-row-header","tag-sql-server","tag-storage-engine"],"acf":[],"yoast_head":"\nThe Row Header (A Storage Wednesday Post) - sqlity.net<\/title>\n<meta name=\"description\" content=\"The row header contains metadata describing the row. 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