Explain trigram index for text indexes (#1153)

Author: JPryce-Aklundh

modules/ROOT/pages/indexes/search-performance-indexes/managing-indexes.adoc

@@ -214,7 +214,7 @@ Instad an informational notification is returned.
 Creating a text index can be done with the `CREATE TEXT INDEX` command.
 Note that the index name must be unique.
 
-Text indexes have no supported index configuration and they have two index providers available, `text-2.0` (default) and `text-1.0` (deprecated).
+Text indexes have no supported index configuration and they have two index providers available, `text-2.0` (default -- see xref:indexes/search-performance-indexes/managing-indexes.adoc#text-indexes-trigram-indexes[Trigram indexing] below for more information) and `text-1.0` (deprecated).
 
 [[text-indexes-supported-predicates]]
 [discrete]
@@ -284,6 +284,18 @@ See the section about xref:indexes/search-performance-indexes/using-indexes.adoc
 [TIP]
 Text indexes are only used for exact query matches. To perform approximate matches (including, for example, variations and typos), and to compute a similarity score between `STRING` values, use semantic xref:indexes/semantic-indexes/full-text-indexes.adoc[full-text indexes] instead.
 
+[[text-indexes-trigram-indexes]]
+==== Trigram indexing
+
+The default text index provider, `text-2.0`, uses trigram indexing.
+This means that `STRING` values are indexed into overlapping trigrams, each containing three Unicode code points.
+For example, the word `"developer"` would be indexed by the following trigrams: `["dev", "eve", "vel", "elo", "lop", "ope", "per"]`.
+
+This makes text indexes particularly suitable for substring (`CONTAINS`) and suffix (`ENDS WITH`) searches, as well as prefix searches (`STARTS WITH`).
+For example, searches like `CONTAINS "vel"` or `ENDS WITH "per"` can be efficiently performed by directly looking up the relevant trigrams in the index.
+By comparison, range indexes, which indexes `STRING` values lexicographically (see xref:indexes/search-performance-indexes/using-indexes.adoc#range-index-backed-order-by[Range index-backed `ORDER BY`] for more information) and are therefore more suited for prefix searches, would need to scan through all indexed values to check if `"vel"` existed anywhere within the text.
+For more information, see xref:indexes/search-performance-indexes/using-indexes.adoc#text-indexes[The impact of indexes on query performance -> Text indexes].
+
 [discrete]
 [[text-indexes-examples]]
 ==== Examples

modules/ROOT/pages/indexes/search-performance-indexes/using-indexes.adoc

@@ -232,7 +232,7 @@ Total database accesses: 7, total allocated memory: 312
 
 This is because range indexes store `STRING` values alphabetically.
 This means that, while they are very efficient for retrieving exact matches of a `STRING`, or for prefix matching, they are less efficient for suffix and contains searches, where they have to scan all relevant properties to filter any matches.
-Text indexes do not store `STRING` properties alphabetically, and are instead optimized for suffix and contains searches.
+Text indexes do not store `STRING` properties alphabetically, and are instead optimized for suffix and contains searches (for more information, see xref:indexes/search-performance-indexes/managing-indexes.adoc#text-indexes-trigram-indexes[Create a text index -> Trigram indexing]).
 That said, if no range index had been present on the name property, the previous query would still have been able to utilize the text index.
 It would have done so less efficiently than a range index, but it still would have been useful.