A field section is similar to a zone section in that it is a region of text delimited by start and end tags. Field sections are more efficient than zone sections and are different than zone sections in that the region is indexed separately from the rest of the document. You can create an unlimited number of field sections.
Since field sections are indexed differently, you can also get better query performance over zone sections for when you have a large number of documents indexed.
Field sections are more suited to when you have a single occurrence of a section in a a document such as a field in a news header. Field sections can also be made visible to the rest of the document.
Unlike zone sections, field sections have the following restrictions:
Field sections cannot overlap
Field sections cannot repeat
Field sections cannot nest
This section contains the following topics.
By default, field sections are indexed as a sub-document separate from the rest of the document. As such, field sections are invisible to the surrounding text and can only be queried by explicitly naming the section in the WITHIN clause.
You can make field sections visible if you want the text within the field section to be indexed as part of the enclosing document. Text within a visible field section can be queried with or without the WITHIN operator.
The following example shows the difference between using invisible and visible field sections.
The following code defines a section group basicgroup of the BASIC_SECTION_GROUP type. It then creates a field section in basicgroup called Author for the <A> tag. It also sets the visible flag to FALSE to create an invisible section:
begin
ctx_ddl.create_section_group('basicgroup', 'BASIC_SECTION_GROUP');
ctx_ddl.add_field_section('basicgroup', 'Author', 'A', FALSE);
end;
Because the Author field section is not visible, to find text within the Author section, you must use the WITHIN operator as follows:
'(Martin Luther King) WITHIN Author'
A query of Martin Luther King without the WITHIN operator does not return instances of this term in field sections. If you want to query text within field sections without specifying WITHIN, you must set the visible flag to TRUE when you create the section as follows:
begin
ctx_ddl.add_field_section('basicgroup', 'Author', 'A', TRUE);
end;
Field sections cannot be nested. For example, if you define a field section to start with <TITLE> and define another field section to start with <FOO>, the two sections cannot be nested as follows:
<TITLE> dog <FOO> cat </FOO> </TITLE>
To work with nested sections, define them as zone sections.
Repeated field sections are allowed, but WITHIN queries treat them as a single section. The following is an example of repeated field section in a document:
<TITLE> cat </TITLE> <TITLE> dog </TITLE>
The query dog and cat within title returns the document, even though these words occur in different sections.
To have WITHIN queries distinguish repeated sections, define them as zone sections.