Wildcard searching settings

Wildcard searching is a common text search type. In Manticore, it is performed at the dictionary level. By default, both plain and RT tables use a dictionary type called dict. In this mode, words are stored as they are, so enabling wildcarding does not affect the size of the table. When a wildcard search is performed, the dictionary is searched to find all possible expansions of the wildcarded word. This expansion can be problematic in terms of computation at query time when the expanded word provides many expansions or expansions that have huge hitlists, especially in the case of infixes where the wildcard is added at the start and end of the word. To avoid such problems, the expansion_limit can be used.

min_prefix_len

min_prefix_len = length

This setting determines the minimum word prefix length to index and search. By default, it is set to 0, meaning prefixes are not allowed.

Prefixes allow for wildcard searching by wordstart* wildcards.

For example, if the word "example" is indexed with min_prefix_len=3, it can be found by searching for "exa", "exam", "examp", "exampl", as well as the full word.

Note that with dict=crc min_prefix_len will affect the size of the full-text index since each word expansion will be stored additionally.

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

Note that with either dict=crc mode or any of the above options disabled, it is not possible to differentiate between prefixes and full words, and perfect word matches cannot be ranked higher.

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

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CREATE TABLE products(title text, price float) min_prefix_len = '3'

min_infix_len

min_infix_len = length

The min_infix_len setting determines the minimum length of an infix prefix to index and search. It is optional and its default value is 0, which means that infixes are not allowed. The minimum allowed non-zero value is 2.

When enabled, infixes allow for wildcard searching with term patterns like start*, *end, *middle*, , and so on. It also allows you to disable too short wildcards if they are too expensive to search for.

If the following conditions are met, Manticore can differentiate perfect word matches from infix matches and rank the former higher:

Note that with the dict=crc mode or any of the above options disabled, there is no way to differentiate between infixes and full words, and thus perfect word matches cannot 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* 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. To limit the impact from wildcard searches with too short wildcards, you might set it higher.

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

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

If dict=keywords, besides 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
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CREATE TABLE products(title text, price float) min_infix_len = '3'

prefix_fields

prefix_fields = field1[, field2, ...]

The prefix_fields setting is used to limit prefix indexing to specific full-text fields in dict=crc mode. By default, all fields are indexed in prefix mode, but because prefix indexing can affect both indexing and searching performance, it may be desired to limit it to certain fields.

To limit prefix indexing to specific fields, use the prefix_fields setting followed by a comma-separated list of field names. If prefix_fields is not set, then all fields will be indexed in prefix mode.

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  • CONFIG
CONFIG
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table products {
  prefix_fields = title, name
  min_prefix_len = 3
  dict = crc

infix_fields

infix_fields = field1[, field2, ...]

The infix_fields setting allows you to specify a list of full-text fields to limit infix indexing to. This applies to dict=crc only and is optional, with the default being to index all fields in infix mode. This setting is similar to prefix_fields, but instead allows you to limit infix indexing to specific fields.

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  • CONFIG
CONFIG
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table products {
  infix_fields = title, name
  min_infix_len = 3
  dict = crc

max_substring_len

max_substring_len = length

The max_substring_len directive sets the maximum substring length to be indexed for either prefix or infix searches. This setting is optional, and its default value is 0 (which means that all possible substrings are indexed). It only applies to dict.

By default, substring indexing in dict indexes all possible substrings as separate keywords, which can result in an overly large full-text index. Therefore, the max_substring_len directive allows you to skip too-long substrings that will probably never be searched for.

For example, a test table of 10,000 blog posts takes up a different amount of 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

Therefore, limiting the max substring length can save 10-15% of the table size.

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

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  • CONFIG
CONFIG
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table products {
  max_substring_len = 12
  min_infix_len = 3
  dict = crc

expand_keywords

expand_keywords = {0|1|exact|star}

This setting expands keywords with their exact forms and/or with stars when possible. The supported values are:

  • 1 - expand to both the exact form and the form with the stars. For instance,running will become (running | *running* | =running)
  • exact - - augment the keyword with only its exact form. For instance, running will become (running | =running)
  • star - augment the keyword by adding * around it. For instance, running will become (running | *running*) This setting is optional, and the default value is 0 (keywords are not expanded).

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.

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CREATE TABLE products(title text, price float) expand_keywords = '1'

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:

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  • expand_keywords_enabled
  • expand_keywords_disabled
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mysql> create table t(f text) min_infix_len='2' expand_keywords='1' morphology='stem_en';
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> insert into t values(1,'running'),(2,'runs'),(3,'runsy');
Query OK, 3 rows affected (0.00 sec)

mysql> select *, weight() from t where match('runs');
+------+---------+----------+
| id   | f       | weight() |
+------+---------+----------+
|    2 | runs    |     1560 |
|    1 | running |     1500 |
|    3 | runsy   |     1500 |
+------+---------+----------+
3 rows in set (0.01 sec)

mysql> drop table t;
Query OK, 0 rows affected (0.01 sec)

mysql> create table t(f text) min_infix_len='2' expand_keywords='exact' morphology='stem_en';
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> insert into t values(1,'running'),(2,'runs'),(3,'runsy');
Query OK, 3 rows affected (0.00 sec)

mysql> select *, weight() from t where match('running');
+------+---------+----------+
| id   | f       | weight() |
+------+---------+----------+
|    1 | running |     1590 |
|    2 | runs    |     1500 |
+------+---------+----------+
2 rows in set (0.00 sec)

This directive does not affect indexer in any way, it only affects searchd.

expansion_limit

expansion_limit = number

Maximum number of expanded keywords for a single wildcard. Details are here.

Low-level tokenization Ignoring stop words