Accepted characters array, with case folding rules. Optional, default values are all characters of most non-CJK languages (non_cjk).

charset_table is a main workhorse of Manticore tokenization process, the process of extracting keywords from document text or query text. It controls what characters are accepted as valid and what are not, and how the accepted characters should be transformed (e.g. should the case be removed or not).

You can think of charset_table as of a big table or array that has a mapping for each and every of 100K+ characters in Unicode. By default, every character maps to 0, which means that it does not occur within keywords and should be treated as a separator. Once mentioned in the table, character is mapped to some other character (most frequently, either to itself or to a lowercase letter), and is treated as a valid keyword part.

The expected value format is a comma-separated list of mappings. Two simplest mappings simply declare a character as valid, and map a single character to another single character, respectively. But specifying the whole table in such form would result in bloated and barely manageable specifications. So there are several syntax shortcuts that let you map ranges of characters at once. The complete list is as follows:

• A->a - Single char mapping, declares source char 'A' as allowed to occur within keywords and maps it to destination char 'a' (but does not declare 'a' as allowed).
• A..Z->a..z - Range mapping, declares all chars in source range as allowed and maps them to the destination range. Does not declare destination range as allowed. Also checks range's lengths (the lengths must be equal).
• a - Stray char mapping, declares a character as allowed and maps it to itself. Equivalent to a->a single char mapping.
• a..z - Stray range mapping, declares all characters in range as allowed and maps them to themselves. Equivalent to a..z->a..z range mapping.
• A..Z/2 - Checkerboard range map. Maps every pair of chars to the second char. More formally, declares odd characters in range as allowed and maps them to the even ones; also declares even characters as allowed and maps them to themselves. For instance, A..Z/2 is equivalent to A->B, B->B, C->D, D->D, ..., Y->Z, Z->Z. This mapping shortcut is helpful for a number of Unicode blocks where uppercase and lowercase letters go in such interleaved order instead of contiguous chunks.

Control characters with codes from 0 to 32 are always treated as separators. Characters with codes 33 to 127, i.e. 7-bit ASCII characters, can be used in the mappings as is. To avoid configuration file encoding issues, 8-bit ASCII characters and Unicode characters must be specified in U+xxx form, where xxx is hexadecimal codepoint number. This form can also be used for 7-bit ASCII characters to encode special ones: e.g. use U+2E to encode dot, U+2C to encode comma. The minimal accepted unicode character code is U+0021.

You can redefine character mapping by specifying it again with another mapping. For example built-in array non_cjk includes characters Ä and ä and maps them both to ascii character a which may not work in some cases (e.g. for the German language). In this case you can redefine the characters so:

charset_table = non_cjk,U+00E4,U+00C4

for case sensitive search or

charset_table = non_cjk,U+00E4,U+00C4->U+00E4

for case insensitive search.

Besides definitions of characters and mappings, there are several built-in aliases that can be used. Current aliases are:

• english
• russian
• non_cjk
• cjk

To be able to work with both cjk and non-cjk languages you should set the options in your configuration file as shown below (with an exception for Chinese):

Blended characters list. Optional, default is empty.

Blended characters are indexed both as separators and valid characters. For instance, assume that & is configured as blended and AT&T occurs in an indexed document. Three different keywords will get indexed, namely at&t, treating blended characters as valid, plus at and t, treating them as separators.

Blended characters should be used carefully:

• since as soon as a character is defined as blended it is not a separator any more which can affect search. For example if you put a comma to the blend_chars and then search for dog,cat it will treat that as a single token dog,cat and if during indexation you didn't index dog,cat as dog,cat, but left only dog cat then it won't be matched.
• therefore you need to make sure that this behaviour is desired and control it with help of the other setting blend_mode

Positions for tokens obtained by replacing blended characters with whitespace are assigned as usual, so regular keywords will be indexed just as if there was no blend_chars specified at all. An additional token that mixes blended and non-blended characters will be put at the starting position. For instance, if AT&T company occurs in the very beginning of the text field, at will be given position 1, t position 2, company position 3, and AT&T will also be given position 1 ("blending" with the opening regular keyword). Thus, querying for either AT&T or just AT will match that document, and querying for "AT T" as a phrase will also match it. Last but not least, phrase query for "AT&T company" will also match it, despite the position.

Blended characters can overlap with special characters used in query syntax (think of T-Mobile or @twitter). Where possible, query parser will automatically handle blended character as blended. For instance, "hello @twitter" within quotes (a phrase operator) would handle @-sign as blended, because @-syntax for field operator is not allowed within phrases. Otherwise, the character would be handled as an operator. So you might want to escape the keywords.

Blended characters can be remapped, so that multiple different blended characters could be normalized into just one base form. This is useful when indexing multiple alternative Unicode codepoints with equivalent glyphs.

Blended tokens indexing mode. Optional, default is trim_none.

By default, tokens that mix blended and non-blended characters get indexed in there entirety. For instance, when both at-sign and an exclamation are in blend_chars, @dude! will get result in two tokens indexed: @dude! (with all the blended characters) and dude (without any). Therefore @dude query will not match it.

blend_mode directive adds flexibility to this indexing behavior. It takes a comma-separated list of options.

Options specify token indexing variants. If multiple options are specified, multiple variants of the same token will be indexed. Regular keywords (resulting from that token by replacing blended with a separator) are always indexed.

• trim_none - Index the entire token
• trim_head - Trim heading blended characters, and index the resulting token
• trim_tail - Trim trailing blended characters, and index the resulting token
• trim_both - Trim both heading and trailing blended characters, and index the resulting token
• trim_all - Trim heading, trailing and middle blended characters, and index the resulting token
• skip_pure - Do not index the token if it's purely blended, that is, consists of blended characters only

Returning to the @dude! example above, setting blend_mode = trim_head, trim_tail will result in two tokens being indexed, @dude and dude!. In this particular example, trim_both would have no effect, because trimming both blended characters results in dude which is already indexed as a regular keyword. Indexing @U.S.A. with trim_both (and assuming that dot is blended two) would result in U.S.A being indexed. Last but not least, skip_pure enables you to fully ignore sequences of blended characters only. For example, one @@@ two would be indexed exactly as one two, and match that as a phrase. That is not the case by default because a fully blended token gets indexed and offsets the second keyword position.

Default behavior is to index the entire token, equivalent to blend_mode = trim_none.

Make sure you undestand that either of the blend modes limits your search, even the default one trim_none as with it and assuming . is a blended char:

• .dog. will become .dog. dog during indexation
• and you won't be able to find it by dog..

The more modes you use, the higher the chance your keyword will match something.

Minimum indexed word length. Optional, default is 1 (index everything).

Only those words that are not shorter than this minimum will be indexed. For instance, if min_word_len is 4, then 'the' won't be indexed, but 'they' will be.

N-gram lengths for N-gram indexing. Optional, default is 0 (disable n-gram indexing). Known values are 0 and 1.

N-grams provide basic CJK (Chinese, Japanese, Korean) support for unsegmented texts. The issue with CJK searching is that there could be no clear separators between the words. In some cases you might not want to use dictionary-based segmentation as the one available for Chinese. In those cases n-gram segmentation might work well too.

When this feature is enabled, streams of CJK (or any other defined in ngram_chars) characters are indexed as N-grams. For example, if incoming text is "ABCDEF" (where A to F represent some CJK characters) and ngram_len is 1, in will be indexed as if it was "A B C D E F". Only ngram_len=1 is supported at the moment. Only those characters that are listed in ngram_chars table will be split this way; other ones will not be affected.

Note that if search query is segmented, i.e. there are separators between individual words, then wrapping the words in quotes and using extended mode will result in proper matches being found even if the text was not segmented. For instance, assume that the original query is BC DEF. After wrapping in quotes on the application side, it should look like "BC" "DEF" (with quotes). This query will be passed to Manticore and internally split into 1-grams too, resulting in "B C" "D E F" query, still with quotes that are the phrase matching operator. And it will match the text even though there were no separators in the text.

Even if the search query is not segmented, Manticore should still produce good results, thanks to phrase based ranking: it will pull closer phrase matches (which in case of N-gram CJK words can mean closer multi-character word matches) to the top.

N-gram characters list. Optional, default is empty.

To be used in conjunction with in ngram_len, this list defines characters, sequences of which are subject to N-gram extraction. Words comprised of other characters will not be affected by N-gram indexing feature. The value format is identical to charset_table. N-gram characters cannot appear in the charset_table.

Also you can use an alias for our default N-gram table as in the example. It should be sufficient in most cases.

Ignored characters list. Optional, default is empty.

Useful in cases when some characters, such as soft hyphenation mark (U+00AD), should be not just treated as separators but rather fully ignored. For example, if '-' is simply not in the charset_table, "abc-def" text will be indexed as "abc" and "def" keywords. On the contrary, if '-' is added to ignore_chars list, the same text will be indexed as a single "abcdef" keyword.

The syntax is the same as for charset_table, but it's only allowed to declare characters, and not allowed to map them. Also, the ignored characters must not be present in charset_table.

Bigram indexing mode. Optional, default is none.

Bigram indexing is a feature to accelerate phrase searches. When indexing, it stores a document list for either all or some of the adjacent words pairs into the index. Such a list can then be used at searching time to significantly accelerate phrase or sub-phrase matching.

bigram_index controls the selection of specific word pairs. The known modes are:

• all, index every single word pair
• first_freq, only index word pairs where the first word is in a list of frequent words (see bigram_freq_words). For example, with bigram_freq_words = the, in, i, a, indexing "alone in the dark" text will result in "in the" and "the dark" pairs being stored as bigrams, because they begin with a frequent keyword (either "in" or "the" respectively), but "alone in" would not be indexed, because "in" is a second word in that pair.
• both_freq, only index word pairs where both words are frequent. Continuing with the same example, in this mode indexing "alone in the dark" would only store "in the" (the very worst of them all from searching perspective) as a bigram, but none of the other word pairs.

For most use cases, both_freq would be the best mode, but your mileage may vary.

A list of keywords considered "frequent" when indexing bigrams. Optional, default is empty.

Some of the bigram indexing modes (see bigram_index) require to define a list of frequent keywords. These are not to be confused with stop words. Stop words are completely eliminated when both indexing and searching. Frequent keywords are only used by bigrams to determine whether to index a current word pair or not.

bigram_freq_words lets you define a list of such keywords.

The keywords dictionary type. Known values are 'crc' and 'keywords'. Optional, default is 'keywords'.

Keywords dictionary mode (dict=keywords), (greatly) reduces indexing impact and enable substring searches on huge collections. That mode is supported both for plain and RT tables.

CRC dictionaries never store the original keyword text in the index. Instead, keywords are replaced with their control sum value (calculated using FNV64) both when searching and indexing, and that value is used internally in the index.

That approach has two drawbacks. First, there is a chance of control sum collision between several pairs of different keywords, growing quadratically with the number of unique keywords in the index. However, it is not a big concern as a chance of a single FNV64 collision in a dictionary of 1 billion entries is approximately 1:16, or 6.25 percent. And most dictionaries will be much more compact that a billion keywords, as a typical spoken human language has in the region of 1 to 10 million word forms.) Second, and more importantly, substring searches are not directly possible with control sums. Manticore alleviated that by pre-indexing all the possible substrings as separate keywords (see min_prefix_len, min_infix_len directives). That actually has an added benefit of matching substrings in the quickest way possible. But at the same time pre-indexing all substrings increases the index size a lot (factors of 3-10x and even more would not be unusual) and impacts the indexing time respectively, rendering substring searches on big indexes rather impractical.

Keywords dictionary fixes both these drawbacks. It stores the keywords in the index and performs search-time wildcard expansion. For example, a search for a 'test*'prefix could internally expand to 'test|tests|testing' query based on the dictionary contents. That expansion is fully transparent to the application, except that the separate per-keyword statistics for all the actually matched keywords would now also be reported.

For substring (infix) search extended wildcards may be used. Special symbols like '?' and '%' are supported along with substring (infix) search (e.g. "t?st*","run%","*abc*"). Note, however, these wildcards work only with dict=keywords, and not elsewhere.

Indexing with keywords dictionary should be 1.1x to 1.3x slower compared to regular, non-substring indexing - but times faster compared to substring indexing (either prefix or infix). Index size should only be slightly bigger that than of the regular non-substring table, with a 1..10% percent total difference. Regular keyword searching time must be very close or identical across all three discussed index kinds (CRC non-substring, CRC substring, keywords). Substring searching time can vary greatly depending on how many actual keywords match the given substring (in other words, into how many keywords does the search term expand). The maximum number of keywords matched is restricted by the expansion_limit directive.

Essentially, keywords and CRC dictionaries represent the two different trade-off substring searching decisions. You can choose to either sacrifice indexing time and index size in favor of top-speed worst-case searches (CRC dictionary), or only slightly impact indexing time but sacrifice worst-case searching time when the prefix expands into very many keywords (keywords dictionary).

Embedded exceptions, wordforms, or stop words file size limit. Optional, default is 16K.

When you create a table the above mentioned files can be either saved externally along with the table or embedded directly into the table. Files sized under embedded_limit get stored into the table. For bigger files, only the file names are stored. This also simplifies moving table files to a different machine; you may get by just copying a single file.

With smaller files, such embedding reduces the number of the external files on which the table depends, and helps maintenance. But at the same time it makes no sense to embed a 100 MB wordforms dictionary into a tiny delta table. So there needs to be a size threshold, and embedded_limit is that threshold.

The path to a file with global (cluster-wide) keyword IDFs. Optional, default is empty (use local IDFs).

On a multi-table cluster, per-keyword frequencies are quite likely to differ across different tables. That means that when the ranking function uses TF-IDF based values, such as BM25 family of factors, the results might be ranked slightly differently depending on what cluster node they reside.

The easiest way to fix that issue is to create and utilize a global frequency dictionary, or a global IDF file for short. This directive lets you specify the location of that file. It is suggested (but not required) to use an .idf extension. When the IDF file is specified for a given table and OPTION global_idf is set to 1, the engine will use the keyword frequencies and collection documents counts from the global_idf file, rather than just the local table. That way, IDFs and the values that depend on them will stay consistent across the cluster.

IDF files can be shared across multiple tables. Only a single copy of an IDF file will be loaded by searchd, even when many tables refer to that file. Should the contents of an IDF file change, the new contents can be loaded with a SIGHUP.

You can build an .idf file using indextool utility, by dumping dictionaries using --dumpdict dict.txt --stats switch first, then converting those to .idf format using --buildidf, then merging all the .idf files across cluster using --mergeidf.

Hitless words list. Optional, allowed values are 'all', or a list file name.

By default, Manticore full-text index stores not only a list of matching documents for every given keyword, but also a list of its in-document positions (known as hitlist). Hitlists enables phrase, proximity, strict order and other advanced types of searching, as well as phrase proximity ranking. However, hitlists for specific frequent keywords (that can not be stopped for some reason despite being frequent) can get huge and thus slow to process while querying. Also, in some cases we might only care about boolean keyword matching, and never need position-based searching operators (such as phrase matching) nor phrase ranking.

hitless_words lets you create indexes that either do not have positional information (hitlists) at all, or skip it for specific keywords.

Hitless index will generally use less space than the respective regular full-text index (about 1.5x can be expected). Both indexing and searching should be faster, at a cost of missing positional query and ranking support.

If used in positional queries (e.g. phrase queries) the hitless words are taken out from them and used as operand without a position. For example if "hello" and "world" are hitless and "simon" and "says" are not hitless, the phrase query "simon says hello world" will be converted to ("simon says" & hello & world), matching "hello" and "world" anywhere in the document and "simon says" as an exact phrase.

A positional query than contains only hitless words will result in an empty phrase node, therefore the entire query will return an empty result and a warning. If the whole dictionary is hitless (using all) only boolean matching can be used on the respective index.

Enables computing and storing of field lengths (both per-document and average per-index values) into the full-text index. Optional, default is 0 (do not compute and store).

When index_field_lengths is set to 1 Manticore will:

• create a respective length attribute for every full-text field, sharing the same name but with __len suffix
• compute a field length (counted in keywords) for every document and store in to a respective attribute
• compute the per-index averages. The lengths attributes will have a special TOKENCOUNT type, but their values are in fact regular 32-bit integers, and their values are generally accessible.

BM25A() and BM25F() functions in the expression ranker are based on these lengths and require index_field_lengths to be enabled. Historically, Manticore used a simplified, stripped-down variant of BM25 that, unlike the complete function, did not account for document length. There's also support for both a complete variant of BM25, and its extension towards multiple fields, called BM25F. They require per-document length and per-field lengths, respectively. Hence the additional directive.

Index-time token filter for full-text indexing. Optional, default is empty.

Index-time token filter gets created by indexer on indexing source data into a plain table or by RT table on processing INSERT or REPLACE statements and lets you implement a custom tokenizer that makes tokens according to custom rules. The plugins are defined as library name:plugin name:optional string of settings.

Position increment on overshort (less than min_word_len) keywords. Optional, allowed values are 0 and 1, default is 1.

Phrase boundary characters list. Optional, default is empty.

This list controls what characters will be treated as phrase boundaries, in order to adjust word positions and enable phrase-level search emulation through proximity search. The syntax is similar to charset_table, but mappings are not allowed and the boundary characters must not overlap with anything else.

On phrase boundary, additional word position increment (specified by phrase_boundary_step) will be added to current word position. This enables phrase-level searching through proximity queries: words in different phrases will be guaranteed to be more than phrase_boundary_step distance away from each other; so proximity search within that distance will be equivalent to phrase-level search.

Phrase boundary condition will be raised if and only if such character is followed by a separator; this is to avoid abbreviations such as S.T.A.L.K.E.R or URLs being treated as several phrases.

Phrase boundary word position increment. Optional, default is 0.

On phrase boundary, current word position will be additionally incremented by this number.

Regular expressions (regexps) to filter the fields and queries with. Optional, multi-value, default is an empty list of regexps.

In certain applications (like product search) there can be many different ways to call a model, or a product, or a property, and so on. For instance, 'iphone 3gs' and 'iphone 3 gs' (or even 'iphone3 gs') are very likely to mean the same product. Or, for a more tricky example, '13-inch', '13 inch', '13"', and '13in' in a laptop screen size descriptions do mean the same.

Regexps provide you with a mechanism to specify a number of rules specific to your application to handle such cases. In the first 'iphone 3gs' example, you could possibly get away with a wordforms files tailored to handle a handful of iPhone models. However even in a comparatively simple second '13-inch' example there is just way too many individual forms and you are better off specifying rules that would normalize both '13-inch' and '13in' to something identical.

Regular expressions listed in regexp_filter are applied in the order they are listed. That happens at the earliest stage possible, before any other processing, even before tokenization. That is, regexps are applied to the raw source fields when indexing, and to the raw search query text when searching.

min_prefix_len = length

Minimum word prefix length to index and search. Optional, default is 0 (do not allow prefixes).

Prefixes allow to implement wildcard searching by wordstart* wildcards.

For instance, if you index word "example" with min_prefix_len=3 you will be able to find it by "exa", "exam", "examp", "exampl" prefixes along with the word itself.

Be aware that in case of dict=crc min_prefix_len will also affect full-text index size as each word expansion will be stored additionally.

Manticore can differentiate perfect word matches from prefix matches and rank the former higher if you conform the following conditions:

• dict=keywords (on by default)
• index_exact_words=1 (off by default),
• expand_keywords=1 (also off by default)

Note that either with the dict=crc mode or with any of the above options disabled, there is no way to differentiate between the prefixes and full words, and thus perfect word matches can't be ranked higher.

When minimum infix length is set to a positive number, minimum prefix length is always considered 1.

min_infix_len = length

Minimum infix prefix length to index and search. Optional, default is 0 (do not allow infixes), and minimum allowed non-zero value is 2.

Infix length setting enables wildcard searches with term patterns like start*, *end, *middle*, and so on. It also lets you disable too short wildcards if those are too expensive to search for.

Manticore can differentiate perfect word matches from infix matches and rank the former higher if you conform the following conditions:

• dict=keywords (on by default)
• index_exact_words=1 (off by default),
• expand_keywords=1 (also off by default)

Note that either with the dict=crc mode or with any of the above options disabled, there is no way to differentiate between the infixes and full words, and thus perfect word matches can't be ranked higher.

Infix wildcard search query time can vary greatly, depending on how many keywords the substring will actually expand to. Short and frequent syllables like *in* or *ti* just might expand to way too many keywords, all of which would need to be matched and processed. Therefore, to generally enable substring searches you would set min_infix_len to 2; and to limit the impact from wildcard searches with too short wildcards, you might set it higher.

Infixes must be at least 2 characters long, wildcards like *a* are not allowed for performance reasons.

When minimum infix length is set to a positive number, minimum prefix length is considered 1. For dict word infixing and prefixing cannot be both enabled at the same. For dict and other fields to have prefixes declared with prefix_fields, but it's forbidden to declare same field in the both lists.

In case of dict=keywords, beside the wildcard * two other wildcard characters can be used:

• ? can match any(one) character: t?st will match test, but not teast
• % can match zero or one character : tes% will match tes or test, but not testing

prefix_fields = field1[, field2, ...]

List of full-text fields to limit prefix indexing to. Applies to dict=crc only. Optional, default is empty (index all fields in prefix mode).

Because prefix indexing impacts both indexing and searching performance, it might be desired to limit it to specific full-text fields only: for instance, to provide prefix searching through URLs, but not through page contents. prefix_fields specifies what fields will be prefix-indexed; all other fields will be indexed in normal mode. The value format is a comma-separated list of field names.

infix_fields = field1[, field2, ...]

The list of full-text fields to limit infix indexing to. Applies to dict=crc only. Optional, default is empty (index all fields in infix mode).

Similar to prefix_fields, but lets you limit infix-indexing to given fields.

max_substring_len = length

Maximum substring (either prefix or infix) length to index. Optional, default is 0 (do not limit indexed substrings). Applies to dict only.

By default, substring (either prefix or infix) indexing in the dict⛔ will index all possible substrings as separate keywords. That might result in an overly large full-text index. So this directive lets you limit the impact of substring indexing by skipping too-long substrings (which, chances are, will never get searched for anyway).

For example, a test table of 10,000 blog posts takes this much disk space depending on the settings:

• 6.4 MB baseline (no substrings)
• 24.3 MB (3.8x) with min_prefix_len = 3
• 22.2 MB (3.5x) with min_prefix_len = 3, max_substring_len = 8
• 19.3 MB (3.0x) with min_prefix_len = 3, max_substring_len = 6
• 94.3 MB (14.7x) with min_infix_len = 3
• 84.6 MB (13.2x) with min_infix_len = 3, max_substring_len = 8
• 70.7 MB (11.0x) with min_infix_len = 3, max_substring_len = 6

So in this test limiting the max substring length saved us 10-15% on the table size.

There is no performance impact associated with substring length when using dict=keywords mode, so this directive is not applicable and intentionally forbidden in that case. If required, you can still limit the length of a substring that you search for in the application code.

expand_keywords = {0|1|exact|star}

Expands keywords with their exact forms (i.e. the forms of the keywords before applying any morphological modifications) and/or stars when possible. The supported values are:

• 1 - expand to both the exact form and the form with the stars. running will become (running | *running* | =running)
• exact - augment the keyword with only its exact form. running will become (running | =running)
• star - augment the keyword by adding * around it. running will become (running | *running*) Optional, default is 0 (do not expand keywords).

Queries against tables with expand_keywords feature enabled are internally expanded as follows: if the table was built with prefix or infix indexing enabled, every keyword gets internally replaced with a disjunction of the keyword itself and a respective prefix or infix (keyword with stars). If the table was built with both stemming and index_exact_words enabled, exact form is also added.

Expanded queries take naturally longer to complete, but can possibly improve the search quality, as the documents with exact form matches should be ranked generally higher than documents with stemmed or infix matches.

Note that the existing query syntax does not allow to emulate this kind of expansion, because internal expansion works on keyword level and expands keywords within phrase or quorum operators too (which is not possible through the query syntax). Take a look at the examples and how expand_keywords affects the search result weights and how "runsy" is found by "runs" w/o the need to add a star:

Stop word files list (space separated). Optional, default is empty. You can specify several file names, separated by spaces. All the files will be loaded. In the RT mode only absolute paths are allowed.

Stop words file format is simple plain text. The encoding must be UTF-8. File data will be tokenized with respect to charset_table settings, so you can use the same separators as in the indexed data.

Stop word files can either be created manually, or semi-automatically. indexer provides a mode that creates a frequency dictionary of the table, sorted by the keyword frequency, see --buildstops and --buildfreqs switch for details. Top keywords from that dictionary can usually be used as stop words.

For example, to use stop words for Italian language just put the following line in your config file:

If you need to use stop words for multiple languages you should list all their aliases, separated with commas (RT mode) or spaces (plain mode):

Position increment on stopwords. Optional, allowed values are 0 and 1, default is 1.

Whether to apply stop words before or after stemming. Optional, default is 0 (apply stop word filter after stemming).

By default, stop words are stemmed themselves, and applied to tokens after stemming (or any other morphology processing). In other words, by default, a token is stopped when stem(token) is equal to stem(stopword). That can lead to unexpected results when a token gets (erroneously) stemmed to a stopped root. For example, 'Andes' might get stemmed to 'and', so when 'and' is a stopword, 'Andes' is also skipped.

stopwords_unstemmed directive changed this behaviour. When it's enabled, stop words are applied before stemming (and therefore to the original word forms), and the tokens are skipped when token is equal to stopword.