Populate Full-Text Indexes

Artikkeli
02/28/2023

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance

Creating and maintaining a full-text index involves populating the index by using a process called a population (also known as a crawl).

Types of population

A full-text index supports the following types of population:

Full population
Automatic or manual population based on change tracking
Incremental population based on a timestamp

Full 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.

On the one hand, 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.
On the other hand, 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 the CREATE FULLTEXT INDEX statement. If you specify CHANGE_TRACKING MANUAL, the Full-Text Engine doesn't populate the new full-text index until you execute an ALTER FULLTEXT INDEX statement using the START FULL POPULATION or START INCREMENTAL POPULATION clause.

Example - Create a full-text index without running a full population

The following example creates a full-text index on the Production.Document table of the AdventureWorks 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

Example - Run a full population on a table

The following example runs a full population on the Production.Document table of the AdventureWorks sample database.

ALTER FULLTEXT INDEX ON Production.Document  
   START FULL POPULATION;

Population based on change tracking

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 you use change tracking, 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 made through WRITETEXT and UPDATETEXT are not reflected in the full-text index, and are not picked up with change tracking.

Note

For tables containing a timestamp column, you can use incremental population instead of change tracking.

When you enable change tracking 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.

Enable change tracking

There are two types of change tracking:

Automatic (CHANGE_TRACKING AUTO option). Automatic change tracking is the default behavior.
Manual (CHANGE_TRACKING MANUAL option).

The type of change tracking determines how the full-text index is populated, as follows:

Automatic population

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 start tracking changes with automatic population
- CREATE FULLTEXT INDEX ... WITH CHANGE_TRACKING AUTO
- ALTER FULLTEXT INDEX ... SET CHANGE_TRACKING AUTO
Example - Alter a full-text index to use automatic change tracking
The following example changes the full-text index of the HumanResources.JobCandidate table of the AdventureWorks sample database to use change tracking with automatic population.
```
USE AdventureWorks;  
GO  
ALTER FULLTEXT INDEX ON HumanResources.JobCandidate SET CHANGE_TRACKING AUTO;  
GO   
```
Manual population

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
Example - Create 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 AdventureWorks sample database.
```
USE AdventureWorks;  
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  
```
Example - Run a manual population
The following example runs a manual population on the change-tracked full-text index of the HumanResources.JobCandidate table of the AdventureWorks sample database.
```
USE AdventureWorks;  
GO  
ALTER FULLTEXT INDEX ON HumanResources.JobCandidate START UPDATE POPULATION;  
GO  
```

Disable change tracking

CREATE FULLTEXT INDEX ... WITH CHANGE_TRACKING OFF
ALTER FULLTEXT INDEX ... SET CHANGE_TRACKING OFF

Incremental population based on a timestamp

An incremental population is an alternative mechanism for manually populating a full-text index. If a table experiences a high volume of inserts, using incremental population can be more efficient that using manual population.

You can run an incremental population for a full-text index that has CHANGE_TRACKING set to MANUAL or OFF.

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.

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. 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 found. This value will be used when the next incremental population starts.

In some cases, the request for an incremental population results in a full population.

A request for incremental population on a table without a timestamp column results in a full population operation.
If the first population on a full-text index is an incremental population, it indexes all rows, making it equivalent to a full population.
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.

Run an incremental population

To run an incremental population, execute an ALTER FULLTEXT INDEX statement using the START INCREMENTAL POPULATION clause.

Create or change a schedule for incremental population

In Management Studio, in Object Explorer, expand the server.
Expand Databases, and then expand the database that contains the full-text index.
Expand Tables.

Right-click the table on which the full-text index is defined, select Full-Text index, and on the Full-Text index context menu, click Properties. This opens the Full-text index Properties dialog box.

Important

If the base table or view does not contain a column of the timestamp data type, incremental population is not possible.
In the Select a page pane, select Schedules.

Use this page to create or manage schedules for a SQL Server Agent job that starts an incremental table population on the base table or indexed view of the full-text index.

The options are as follows:
- To create a new schedule, click New.
  
  This opens the New Full-Text Indexing Table Schedule dialog box, where you can create a schedule. To save the schedule, click OK.
  
  Important
  
  A SQL Server Agent job (Start Incremental Table Population on database_name.table_name) is associated with a new schedule after you exit the Full-Text Index Properties dialog box. If you create multiple schedules for the same full-text index, they all use the same job.
- To change an existing schedule, select the existing schedule and click Edit.
  
  This opens the New Full-Text Indexing Table Schedule dialog box, where you can modify the schedule.
  
  Note
  
  For information about modifying a SQL Server Agent job, see Modify a Job.
- To remove an existing schedule, select the existing schedule and click Delete.
Select OK.

Troubleshoot errors in a full-text population (crawl)

When an error occurs during a crawl, the Full-Text Search crawl logging facility creates and maintains a crawl log, which is a plain text file. Each crawl log corresponds to a particular full-text catalog. By default, crawl logs for a given instance (in this example, the default instance) are located in %ProgramFiles%\Microsoft SQL Server\MSSQL15.MSSQLSERVER\MSSQL\LOG folder.

The crawl log file follows the following naming scheme:

SQLFT<DatabaseID><FullTextCatalogID>.LOG[<n>]

The variable parts of the crawl log file name are the following.

<DatabaseID> - The ID of a database. <dbid> is a five digit number with leading zeros.
<FullTextCatalogID> - Full-text catalog ID. <catid> is a five digit number with leading zeros.
<n> - Is an integer that indicates one or more crawl logs of the same full-text catalog exist.

For example, SQLFT0000500008.2 is the crawl log file for a database with database ID = 5, and full-text catalog ID = 8. The 2 at the end of the file name indicates that there are two crawl log files for this database/catalog pair.

Jaa