Full-Text Index Population
Creating and maintaining a full-text index involves populating the index by using a process called a population (also known as a crawl). SQL Server supports the following types of population: full population, change tracking-based automatic or manual population, and incremental timestamp-based population.
During a full population, index entries are built for all the rows of a table or indexed view. A full population of a full-text index, builds index entries for all the rows of the base table or indexed view.
By default, SQL Server populates a new full-text index fully as soon as it is created. However, a full population can consume a significant amount of resources. Therefore, when creating a full-text index during peak periods, it is often a best practice to delay the full population until an off-peak time, particularly if the base table of an full-text index is large. However, the full-text catalog to which the index belongs is not usable until all of its full-text indexes are populated. To create a full-text index without populating it immediately, specify the CHANGE_TRACKING OFF, NO POPULATION clause in your CREATE FULLTEXT INDEX Transact-SQL statement. SQL Server will not populate the new full-text index until you execute an ALTER FULLTEXT INDEX Transact-SQL statement using the START FULL POPULATION or START INCREMENTAL POPULATION clause. For more information, see examples "A. Creating a full-text index without running a full population" and "B. Running a full population on table," later in this topic.
Optionally, you can use change tracking to maintain a full-text index after its initial full population. There is a small overhead associated with change tracking because SQL Server maintains a table in which it tracks changes to the base table since the last population. When change tracking is used, SQL Server maintains a record of the rows in the base table or indexed view that have been modified by updates, deletes, or inserts. Data changes through WRITETEXT and UPDATETEXT are not reflected in the full-text index, and are not picked up with change tracking.
For tables containing a timestamp column, you can use incremental populations.
When change tracking is enabled during index creation, SQL Server fully populates the new full-text index immediately after it is created. Thereafter, changes are tracked and propagated to the full-text index. There are two types of change tracking, automatic (CHANGE_TRACKING AUTO option) and manual (CHANGE_TRACKING MANUAL option). Automatic change tracking is the default behavior.
The type of change tracking determines how the full-text index is populated, as follows:
By default, or if you specify CHANGE_TRACKING AUTO, the Full-Text Engine uses automatic population on the full-text index. After the initial full population completes, changes are tracked as data is modified in the base table, and the tracked changes are propagated automatically. The full-text index is updated in the background, however, so propagated changes might not be reflected immediately in the index.
To set up tracking changes with automatic population
CREATE FULLTEXT INDEX … WITH CHANGE_TRACKING AUTO
ALTER FULLTEXT INDEX … SET CHANGE_TRACKING AUTO
For more information, see example "E. Altering a full-text index to use automatic change tracking," later in this topic.
If you specify CHANGE_TRACKING MANUAL, the Full-Text Engine uses manual population on the full-text index. After the initial full population completes, changes are tracked as data is modified in the base table. However, they are not propagated to the full-text index until you execute an ALTER FULLTEXT INDEX … START UPDATE POPULATION statement. You can use SQL Server Agent to call this Transact-SQL statement periodically.
To start tracking changes with manual population
CREATE FULLTEXT INDEX … WITH CHANGE_TRACKING MANUAL
ALTER FULLTEXT INDEX … SET CHANGE_TRACKING MANUAL
For more information, see examples "C. Creating a full-text index with manual change tracking" and "D. Running a manual population," later in this topic.
To turn off change tracking
CREATE FULLTEXT INDEX … WITH CHANGE_TRACKING OFF
ALTER FULLTEXT INDEX … SET CHANGE_TRACKING OFF
An incremental population is an alternative mechanism for manually populating a full-text index. You can run an incremental population for a full-text index that has CHANGE_TRACKING set to MANUAL or OFF. If the first population on a full-text index is an incremental population, it indexes all rows, making it equivalent to a full population.
The requirement for incremental population is that the indexed table must have a column of the timestamp data type. If a timestamp column does not exist, incremental population cannot be performed. A request for incremental population on a table without a timestamp column results in a full population operation. Also, if any metadata that affects the full-text index for the table has changed since the last population, incremental population requests are implemented as full populations. This includes metadata changes caused by altering any column, index, or full-text index definitions.
SQL Server uses the timestamp column to identify rows that have changed since the last population. The incremental population then updates the full-text index for rows added, deleted, or modified after the last population, or while the last population was in progress. If a table experiences a high volume of inserts, using incremental population can be more efficient that using manual population.
At the end of a population, the Full-Text Engine records a new timestamp value. This value is the largest timestamp value that SQL Gatherer has encountered. This value will be used when a subsequent incremental population starts.
To run an incremental population, execute an ALTER FULLTEXT INDEX statement using the START INCREMENTAL POPULATION clause.
To schedule an incremental population job
The examples in this section use the Production.Document or HumanResources.JobCandidate table of the AdventureWorks2008R2 sample database.
A. Creating a full-text index without running a full population
The following example creates a full-text index on the Production.Document table of the AdventureWorks2008R2 sample database. This example uses WITH CHANGE_TRACKING OFF, NO POPULATION to delay the initial full population.
CREATE UNIQUE INDEX ui_ukDoc ON Production.Document(DocumentID); CREATE FULLTEXT CATALOG AW_Production_FTCat; CREATE FULLTEXT INDEX ON Production.Document ( Document --Full-text index column name TYPE COLUMN FileExtension --Name of column that contains file type information Language 1033 --1033 is LCID for the English language ) KEY INDEX ui_ukDoc ON AW_Production_FTCat WITH CHANGE_TRACKING OFF, NO POPULATION; GO
B. Running a full population on table
The following example runs a full population on the Production.Document table of the AdventureWorks2008R2 sample database.
ALTER FULLTEXT INDEX ON Production.Document START FULL POPULATION;
C. Creating a full-text index with manual change tracking
The following example creates a full-text index that will use change tracking with manual population on the HumanResources.JobCandidate table of the AdventureWorks2008R2 sample database.
USE AdventureWorks2008R2; GO CREATE UNIQUE INDEX ui_ukJobCand ON HumanResources.JobCandidate(JobCandidateID); CREATE FULLTEXT CATALOG ft AS DEFAULT; CREATE FULLTEXT INDEX ON HumanResources.JobCandidate(Resume) KEY INDEX ui_ukJobCand WITH CHANGE_TRACKING=MANUAL; GO
D. Running a manual population
The following example runs a manual population on the change-tracked full-text index of the HumanResources.JobCandidate table of the AdventureWorks2008R2 sample database.
USE AdventureWorks2008R2; GO ALTER FULLTEXT INDEX ON HumanResources.JobCandidate START UPDATE POPULATION; GO
E. Altering a full-text index to use automatic change tracking
The following example changes the full-text index of the HumanResources.JobCandidate table of the AdventureWorks2008R2 sample database to use change tracking with automatic population.
USE AdventureWorks2008R2; GO ALTER FULLTEXT INDEX ON HumanResources.JobCandidate SET CHANGE_TRACKING AUTO; GO