Stopword Transformations in Oracle Text

H Stopword Transformations in Oracle Text

This appendix describes the stopword rewrites or transformations for each operator. In all tables, the Stopword Expression column describes the query expression or component of a query expression, while the right-hand column describes the way Oracle Text rewrites the query.

This appendix contains the following topics:

H.1 Understanding Stopword Transformations

When you use a stopword or stopword-only phrase as an operand for a query operator, Oracle Text rewrites the expression to eliminate the stopword or stopword-only phrase and then executes the query.

The token stopword stands for a single stopword or a stopword-only phrase.

The token non_stopword stands for either a single non-stopword, a phrase of all non-stopwords, or a phrase of non-stopwords and stopwords.

The token no_lex stands for a single character or a string of characters that is neither a stopword nor a word that is indexed. For example, the + character by itself is an example of a no_lex token.

When the Stopword Expression column completely describes the query expression, a rewritten expression of no_token means that no hits are returned when you enter such a query.

When the Stopword Expression column describes a component of a query expression with more than one operator, a rewritten expression of no_token means that a no_token value is passed to the next step of the rewrite.

Transformations that contain a no_token as an operand in the Stopword Expression column describe intermediate transformations in which the no_token is a result of a previous transformation. These intermediate transformations apply when the original query expression has at least one stopword and more than one operator.

For example, consider the following compound query expression:

'(this NOT dog) AND cat'

Assuming that this is the only stopword in this expression, Oracle Text applies the following transformations in the following order:

stopword NOT non-stopword => no_token

no_token AND non_stopword => non_stopword

The resulting expression is:

'cat'

H.2 About Stopwords in Phrase Queries

If used in a phrase query, a stopword will match any single word, whether that word is a stopword or not. For example, if "in" and "to" are stopwords, but "throughout" is not, then the query "hiking in California" will match any of these phrases:

hiking in California
hiking to California
hiking throughout California

H.3 Word Transformations

Stopword Expression	Rewritten Expression
stopword	no_token
no_lex	no_token

The first transformation means that a stopword or stopword-only phrase by itself in a query expression results in no hits.

The second transformation says that a term that is not lexed, such as the + character, results in no hits.

H.4 AND Transformations

Stopword Expression	Rewritten Expression
non_stopword AND stopword	non_stopword
non_stopword AND no_token	non_stopword
stopword AND non_stopword	non_stopword
no_token AND non_stopword	non_stopword
stopword AND stopword	no_token
no_token AND stopword	no_token
stopword AND no_token	no_token
no_token AND no_token	no_token

H.5 OR Transformations

Stopword Expression	Rewritten Expression
non_stopword OR stopword	non_stopword
non_stopword OR no_token	non_stopword
stopword OR non_stopword	non_stopword
no_token OR non_stopword	non_stopword
stopword OR stopword	no_token
no_token OR stopword	no_token
stopword OR no_token	no_token
no_token OR no_token	no_token

H.6 ACCUMulate Transformations

Stopword Expression	Rewritten Expression
non_stopword ACCUM stopword	non_stopword
non_stopword ACCUM no_token	non_stopword
stopword ACCUM non_stopword	non_stopword
no_token ACCUM non_stopword	non_stopword
stopword ACCUM stopword	no_token
no_token ACCUM stopword	no_token
stopword ACCUM no_token	no_token
no_token ACCUM no_token	no_token

H.7 MINUS Transformations

Stopword Expression	Rewritten Expression
non_stopword MINUS stopword	non_stopword
non_stopword MINUS no_token	non_stopword
stopword MINUS non_stopword	no_token
no_token MINUS non_stopword	no_token
stopword MINUS stopword	no_token
no_token MINUS stopword	no_token
stopword MINUS no_token	no_token
no_token MINUS no_token	no_token

H.8 MNOT Transformations

Stopword Expression	Rewritten Expression
non_stopword MNOT stopword	non_stopword
non_stopword MNOT no_token	non_stopword
stopword MNOT non_stopword	no_token
no_token MNOT non_stopword	no_token
stopword MNOT stopword	no_token
no_token MNOT stopword	no_token
stopword MNOT no_token	no_token
no_token MNOT no_token	no_token

H.9 NOT Transformations

Stopword Expression	Rewritten Expression
non_stopword NOT stopword	non_stopword
non_stopword NOT no_token	non_stopword
stopword NOT non_stopword	no_token
no_token NOT non_stopword	no_token
stopword NOT stopword	no_token
no_token NOT stopword	no_token
stopword NOT no_token	no_token
no_token NOT no_token	no_token

H.10 EQUIValence Transformations

Stopword Expression	Rewritten Expression
non_stopword EQUIV stopword	non_stopword
non_stopword EQUIV no_token	non_stopword
stopword EQUIV non_stopword	non_stopword
no_token EQUIV non_stopword	non_stopword
stopword EQUIV stopword	no_token
no_token EQUIV stopword	no_token
stopword EQUIV no_token	no_token
no_token EQUIV no_token	no_token

Note:

When you use query explain plan, not all of the equivalence transformations are represented in the EXPLAIN table.

H.11 NEAR Transformations

Stopword Expression	Rewritten Expression
non_stopword NEAR stopword	non_stopword
non_stopword NEAR no_token	non_stopword
stopword NEAR non_stopword	non_stopword
no_token NEAR non_stopword	non_stopword
stopword NEAR stopword	no_token
no_token NEAR stopword	no_token
stopword NEAR no_token	no_token
no_token NEAR no_token	no_token

H.12 Weight Transformations

Stopword Expression	Rewritten Expression
stopword * n	no_token
no_token * n	no_token

H.13 Threshold Transformations

Stopword Expression	Rewritten Expression
stopword > n	no_token
no_token > n	no_token

H.14 WITHIN Transformations

Stopword Expression	Rewritten Expression
stopword WITHIN section	no_token
no_token WITHIN section	no_token