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Text of the page (random words):
ained within the database data itself indexes to persist interesting orders of the data and other information this information is intermixed within the database file but efforts are made to keep data used together clustered together within the database an ese database may contain up to 2 32 pages or 16 terabytes of data 3 for 8 kilobyte sized pages ese databases are organized into groups called instances most applications use a single instance but all applications can also use multiple instances the importance of the instance is that it associates a single recovery log series with one or more databases currently up to 6 user databases may be attached to an ese instance at any time each separate process using ese may have up to 1024 ese instances a database is portable in that it can be detached from one running ese instance and later attached to the same or a different running instance while detached a database may be copied using standard windows utilities the database cannot be copied while it is being actively used since ese opens database files exclusively a database may physically reside on any device supported for directly addressable i o operations by windows tables edit a table is a homogeneous collection of records where each record has the same set of columns each table is identified by a table name whose scope is local to the database in which the table is contained the amount of disk space allocated to a table within a database is determined by a parameter given when the table is created with the createtable operation tables grow automatically in response to data creation tables have one or more indexes there must be at least one clustered index for record data when no clustered index is defined by the application an artificial index is used which orders and clusters records by the chronological order of record insertion indexes are defined to persist interesting orders of data and allow both sequential access to records in index order and direct access to records by index column values clustered indexes in ese must also be primary meaning that the index key must be unique clustered and non clustered indexes are represented using b trees if an insert or update operation causes a page to overflow the page is split a new page is allocated and is logically chained in between the two previously adjacent pages since this new page is not physically adjacent to its logical neighbors access to it is not as efficient ese has an on line compaction feature that re compacts data if a table is expected to be frequently updated space may be reserved for future insertions by specifying an appropriate page density when creating a table or index this allows split operations to be avoided or postponed records and columns edit a record is an associated set of column values records are inserted and updated via update operations and can be deleted via delete operations columns are set and retrieved via setcolumns and retrievecolumns operations respectively the maximum size of a record is 8110 bytes for 8 kilobyte pages with the exception of long value columns column types of longtext and longbinary do not contribute significantly to this size limitation and records can hold data much larger than a database page size when data is stored in long value columns when a long value reference is stored in a record only 9 bytes of in record data are required these long values may themselves be up to 2 gigabytes gb in size records are typically uniform in that each record has a set of values for the same set of columns in ese it is also possible to define many columns for a table and yet have any given record contain only a small number of non null column values in this sense a table can also be a collection of heterogeneous records ese supports a wide range of columns values ranging in size from 1 bit to 2 gb choosing the correct column type is important because the type of a column determines many of its properties including its ordering for indexes the following data types are supported by ese column types edit name description bit ternary value null 0 or 1 unsigned byte 1 byte unsigned integer short 2 byte signed integer unsigned short 2 byte unsigned integer long 4 byte signed integer unsigned long 4 byte unsigned integer longlong 8 byte signed integer unsignedlonglong 8 byte unsigned integer currency 8 byte signed integer ieee single 4 byte floating point number ieee double 8 byte floating point number datetime 8 byte date time integral date fractional time guid 16 byte unique identifier binary binary string length 255 bytes text ansi or unicode string length 255 bytes long binary large binary string length 2 gb long text large ansi or unicode string length 2 gb fixed variable and tagged columns edit each ese table can define up to 127 fixed length columns 128 variable length columns and 64 993 tagged columns fixed columns are essentially columns that take up the same amount of space in each record regardless of their value fixed columns take up a 1 bit to represent nullity of the column value and a fixed amount of space in each record in which that column or a later defined fixed column is set variable columns are essentially columns that take up a variable amount of space in each record in which they are set depending upon the size of the particular column value variable columns take up 2 bytes to determine nullity and size and a variable amount of space in each record in which that column is set tagged columns are columns that take no space whatsoever if they are not set in a record they may be single valued but can also be multi valued the same tagged column may have multiple values in a single record when tagged columns are set in a record each instance of a tagged column takes approximately 4 bytes of space in addition to the size of the tagged column instance value when the number of instances of a single tagged column is large the overhead per tagged column instance is approximately 2 bytes tagged columns are ideal for sparse columns because they take no space whatsoever if they are not set if a multi valued tagged column is indexed the index will contain one entry for the record for each value of the tagged column for a given table columns fall into one of two categories those which either occur exactly once in each of the records with possibly a few null values and those which occur rarely or which may have multiple occurrences in a single record fixed and variable columns belong to the former category while tagged columns belong to the latter the internal representation of the two column categories is different and it is important to understand the trade offs between the column categories fixed and variable columns are typically represented in every record even when the occurrence has a null value these columns can be quickly addressed via an offset table tagged column occurrences are preceded by a column identifier and the column is located by binary searching the set of tagged columns long values edit column types of long text and long binary are large binary objects they are stored in separate b tree from the clustered index keyed by long value id and byte offset ese supports append byte range overwrite and set size for these columns also ese has a single instance store feature where multiple records may reference the same large binary object as though each record had its own copy of the information i e without inter record locking conflicts the maximum size of a long text or long binary column value is 2 gb version auto increment and escrow columns edit version columns are automatically incremented by ese each time a record containing this column is modified via an update operation this column cannot be set by the application but can only be read applications of version columns include being used to determine if an in memory copy of a given record needs to be refreshed if the value in a table record is greater than the value in a cached copy then the cached copy is known to be out of date version columns must be of type long auto increment columns are automatically set by ese such that the value contained in the column is unique for every record in the table these columns like version columns cannot be set by the application auto increment columns are read only and are automatically set when a new record is inserted into a table via an update operation the value in the column remains constant for the life of the record and only one auto increment column is allowed per table auto increment columns may be of type long or type currency escrow columns can be modified via an escrowupdate operation escrowed updates are numeric delta operations escrow columns must be of type long examples of numeric delta operations include adding 2 to a value or subtracting 1 from a value ese tracks the change in a value rather than the end value of an update multiple sessions may each have outstanding changes made via escrowupdate to the same value because ese can determine the actual end value regardless of which transactions commit and which transactions rollback this allows multiple users to concurrently update a column by making numeric delta changes optionally database engine can erase records with zero value of the column a common use for such escrow column is reference counter many threads increment decrement the value without locks and when the counter reaches zero the record automatically gets deleted indexes edit an index is a persisted ordering of records in a table indexes are used for both sequential access to rows in the order defined and for direct record navigation based on indexed column values the order defined by an index is described in terms of an array of columns in precedence order this array of columns is also called the index key each column is called an index segment each index segment may be either ascending or descending in terms of its ordering contribution any number of indexes may be defined for a table ese provides a rich set of indexing features clustered indexes edit one index may be specified as the clustered or primary index in ese the clustered index must be unique and is referred to as the primary index other indexes are described as non clustered or secondary indexes primary indexes are different from secondary indexes in that the index entry is the record itself and not a logical pointer to the record secondary indexes have primary keys at their leaves to logically link to the record in the primary index in other words the table is physically clustered in primary index order retrieval of non indexed record data in primary index order is generally much faster than in secondary index order this is because a single disk access can bring into memory multiple records that will be access close together in time the same disk access satisfies multiple record access operations however the insertion of a record into the middle of an index as determined by the primary index order may be very much slower than appending it to the end of an index update frequency must be carefully considered against retrieval patterns when performing table design if no primary index is defined for a table then an implicit primary index called a database key dbk index is created the dbk is simply a unique ascending number incremented each time a record is inserted as a result the physical order of records in a dbk index is chronological insertion order and new records are always added at the end of the table if an application wishes to cluster data on a non unique index this is possible by adding an autoincrement column to the end of the non unique index definition indexing over multi valued columns edit indexes can be defined over multi valued columns multiple entries may exist in these indexes for records with multiple values for the indexed column multi valued columns may be indexed in conjunction with single valued columns when two or more multi valued columns are indexed together then the multi valued property is only honored for the first multi value column in the index lower precedence columns are treated as though they were single valued sparse indexes edit indexes can also be defined to be sparse sparse indexes do not have at least one entry for each record in the table there are a number of options in defining a sparse index options exist to exclude records from indexes when an entire index key is null when any key segment is null or when just the first key segment is null indexes can also have conditional columns these columns never appear within an index but can cause a record not to be indexed when the conditional column is either null or non null tuple indexes edit indexes can also be defined to include one entry for each sub string of a text or long text column these indexes are called tuple indexes they are used to speed queries with sub string matching predicates tuple indexes can only be defined for text columns for example if a text column value is i love jet blue and the index is configured to have a minimum tuple size of 4 characters and a maximum tuple length of 10 characters then the following sub strings will be indexed i love jet love jet love jet b ove jet bl ve jet blu e jet blue jet blue jet blue et blue t blue blue blue even though tuple indexes can be very large they can significantly speed queries of the form find all records containing jet blue they can be used for sub strings longer than the maximum tuple length by dividing the search sub string into maximum tuple length search strings and intersecting the results they can be used for exact matches for strings as long as the maximum tuple length or as short as the minimum tuple length with no index intersection for more information on performing index intersection in ese see index intersection tuple indexes cannot speed queries where the search string is shorter than the minimum tuple length transactions edit a transaction is a logical unit of processing delimited by begintransaction and committransaction or rollback operations all updates performed during a transaction are atomic they either all appear in the database at the same time or none appear any subsequent updates by other transactions are invisible to a transaction however a transaction can update only data that has not changed in the meantime else the operation fails at once without waiting read only transactions never need to wait and update transactions can interfere only with one another updating transaction transactions which are terminated by rollback or by a system crash leave no trace on the database in general the data state is restored on rollback to what it was prior to begintransaction transactions may be nested up to 7 levels with one additional level reserved for ese internal use this means that a part of a transaction may be rolled back without need to roll back the entire transaction a committransaction of a nested transaction merely signifies the success of one ph...
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