Initial inverted index docs

docs/en/engines/table-engines/mergetree-family/invertedindexes.md

@@ -0,0 +1,66 @@
+# Inverted indexes [experimental] {#table_engines-ANNIndex}
+
+Inverted indexes are an experimental type of [secondary indexes](mergetree.md#available-types-of-indices) which provide fast text search
+capabilities for [String](../../../sql-reference/data-types/string.md) or [FixedString](../../../sql-reference/data-types/fixedstring.md)
+columns. The main idea of an inverted indexes is to store a mapping from "terms" to the rows which contains these terms. "Terms" are
+tokenized cells of the string column. For example, string cell "I will be a little late" is by default tokenized into six terms "I", "will",
+"be", "a", "little" and "late". Another kind of tokenizer are n-grams. For example, the result of 3-gram tokenization will be 21 terms "I w",
+" wi", "wil", "ill", "ll ", "l b", " be" etc. The more fine-granular the input strings are tokenized, the bigger but also the more
+useful the resulting inverted index will be. 
+
+:::warning
+Inverted indexes are experimental and should not be used in production environment yet. They may change in future in backwards-incompatible
+ways, for example with respect to their DDL/DQL syntax or performance/compression characteristics.
+:::
+
+## Usage
+
+To use inverted indexes, first enable them in the configuration:
+
+```sql
+SET allow_experimental_inverted_index = true;
+```
+
+An inverted index can be defined on a string column using the following syntax
+
+``` sql
+CREATE TABLE tab (key UInt64, str String, INDEX inv_idx(s) TYPE inverted(N) GRANULARITY 1) Engine=MergeTree ORDER BY (k);
+```
+
+where `N` specifies the tokenizer:
+
+- `inverted(0)` (or shorter: `inverted()`) set the tokenizer to "tokens", i.e. split strings along spaces,
+- `inverted(N)` with `N` between 2 and 8 sets the tokenizer to "ngrams(N)"
+
+Being a type of skipping indexes, inverted indexes can be dropped or added to a column after table creation:
+
+``` sql
+ALTER TABLE tbl DROP INDEX inv_idx;
+ALTER TABLE tbl ADD INDEX inv_idx(s) TYPE inverted(2) GRANULARITY 1;
+```
+
+To use the index, no special functions or syntax are required. Typical string search predicates automatically leverage the index. As
+examples, consider:
+
+```sql
+SELECT * from tab WHERE s == 'Hello World;;
+SELECT * from tab WHERE s IN (‘Hello’, ‘World’);
+SELECT * from tab WHERE s LIKE ‘%Hello%’;
+SELECT * from tab WHERE multiSearchAny(s, ‘Hello’, ‘World’);
+SELECT * from tab WHERE hasToken(s, ‘Hello’);
+SELECT * from tab WHERE multiSearchAll(s, [‘Hello’, ‘World’])
+```
+
+The inverted index also works on columns of type `Array(String)`, `Array(FixedString)`, `Map(String)` and `Map(String)`.
+
+Like for other secondary indices, each column part has its own inverted index. Furthermore, each inverted index is internally divided into
+"segments". The existence and size of the segments is generally transparent to users but the segment size determines the memory consumption
+during index construction (e.g. when two parts are merged). Configuration parameter "max_digestion_size_per_segment" (default: 256 MB)
+controls the amount of data read consumed from the underlying column before a new segment is created. Incrementing the parameter raises the
+intermediate memory consumption for index constuction but also improves lookup performance since fewer segments need to be checked on
+average to evaluate a query.
+
+Unlike other secondary indices, inverted indexes (for now) map to row numbers (row ids) instead of granule ids. The reason for this design
+is performance. In practice, users often search for multiple terms at once. For example, filter predicate `WHERE s LIKE '%little%' OR s LIKE
+'%big%'` can be evaluated directly using an inverted index by forming the union of the rowid lists for terms "little" and "big". This also
+means that parameter `GRANULARITY` supplied to index creation has no meaning (it may be removed from the syntax in future).

docs/en/engines/table-engines/mergetree-family/mergetree.md

@@ -428,6 +428,7 @@ Syntax: `tokenbf_v1(size_of_bloom_filter_in_bytes, number_of_hash_functions, ran
 #### Special-purpose
 
 - An experimental index to support approximate nearest neighbor (ANN) search. See [here](annindexes.md) for details.
+- An experimental inverted index to support full-text search. See [here](invertedindexes.md) for details.
 
 ## Example of index creation for Map data type