Query cache

Query cache stores a compressed result set in memory, and then reuses it for subsequent queries where possible. You can configure it using the following directives:

  • qcache_max_bytes, a limit on the RAM use for cached queries storage. Defaults to 16 MB. Setting qcache_max_bytes to 0 completely disables the query cache.
  • qcache_thresh_msec, the minimum wall query time to cache. Queries that completed faster than this will not be cached. Defaults to 3000 msec, or 3 seconds.
  • qcache_ttl_sec, cached entry TTL, or time to live. Queries will stay cached for this much. Defaults to 60 seconds, or 1 minute.

These settings can be changed on the fly using the SET GLOBAL statement:

mysql> SET GLOBAL qcache_max_bytes=128000000;

These changes are applied immediately, and the cached result sets that no longer satisfy the constraints are immediately discarded. When reducing the cache size on the fly, MRU (most recently used) result sets win.

Query cache works as follows. When it's enabled, every full-text search result gets completely stored in memory. That happens after full-text matching, filtering, and ranking, so basically we store total_found {docid,weight} pairs. Compressed matches can consume anywhere from 2 bytes to 12 bytes per match on average, mostly depending on the deltas between the subsequent docids. Once the query completes, we check the wall time and size thresholds, and either save that compressed result set for reuse, or discard it.

Note how the query cache impact on RAM is thus not limited by qcache_max_bytes! If you run, say, 10 concurrent queries, each of them matching upto 1M matches (after filters), then the peak temporary RAM use will be in the 40 MB to 240 MB range, even if in the end the queries are quick enough and do not get cached.

Queries can then use cache when the index, the full-text query (ie.MATCH() contents), and the ranker are all a match, and filters are compatible. Meaning:

  • The full-text part within MATCH() must be a bytewise match. Add a single extra space, and that is now a different query where the query cache is concerned.
  • The ranker (and its parameters if any, for user-defined rankers) must be a bytewise match.
  • The filters must be a superset of the original filters. That is, you can add extra filters and still hit the cache. (In this case, the extra filters will be applied to the cached result.) But if you remove one, that will be a new query again.

Cache entries expire with TTL, and also get invalidated on index rotation, or on TRUNCATE, or on ATTACH. Note that at the moment entries are not invalidated on arbitrary RT index writes! So a cached query might be returning older results for the duration of its TTL.

Current cache status can be inspected with in SHOW STATUS through the qcache_XXX variables:

mysql> SHOW STATUS LIKE 'qcache%';
+-----------------------+----------+
| Counter               | Value    |
+-----------------------+----------+
| qcache_max_bytes      | 16777216 |
| qcache_thresh_msec    | 3000     |
| qcache_ttl_sec        | 60       |
| qcache_cached_queries | 0        |
| qcache_used_bytes     | 0        |
| qcache_hits           | 0        |
+-----------------------+----------+
6 rows in set (0.00 sec)

Collations

Collations essentially affect the string attribute comparisons. They specify both the character set encoding and the strategy that Manticore uses to compare strings when doing ORDER BY or GROUP BY with a string attribute involved.

String attributes are stored as is when indexing, and no character set or language information is attached to them. That's okay as long as Manticore only needs to store and return the strings to the calling application verbatim. But when you ask Manticore to sort by a string value, that request immediately becomes quite ambiguous.

First, single-byte (ASCII, or ISO-8859-1, or Windows-1251) strings need to be processed differently that the UTF-8 ones that may encode every character with a variable number of bytes. So we need to know what is the character set type to interpret the raw bytes as meaningful characters properly.

Second, we additionally need to know the language-specific string sorting rules. For instance, when sorting according to US rules in en_US locale, the accented character ï (small letter i with diaeresis) should be placed somewhere after z. However, when sorting with French rules and fr_FR locale in mind, it should be placed between i and j. And some other set of rules might choose to ignore accents at all, allowing ï and i to be mixed arbitrarily.

Third, but not least, we might need case-sensitive sorting in some scenarios and case-insensitive sorting in some others.

Collations combine all of the above: the character set, the language rules, and the case sensitivity. Manticore currently provides the following four collations.

  1. libc_ci
  2. libc_cs
  3. utf8_general_ci
  4. binary

The first two collations rely on several standard C library (libc) calls and can thus support any locale that is installed on your system. They provide case-insensitive (_ci) and case-sensitive (_cs) comparisons respectively. By default they will use C locale, effectively resorting to bytewise comparisons. To change that, you need to specify a different available locale using collation_libc_locale directive. The list of locales available on your system can usually be obtained with the locale command:

$ locale -a
C
en_AG
en_AU.utf8
en_BW.utf8
en_CA.utf8
en_DK.utf8
en_GB.utf8
en_HK.utf8
en_IE.utf8
en_IN
en_NG
en_NZ.utf8
en_PH.utf8
en_SG.utf8
en_US.utf8
en_ZA.utf8
en_ZW.utf8
es_ES
fr_FR
POSIX
ru_RU.utf8
ru_UA.utf8

The specific list of the system locales may vary. Consult your OS documentation to install additional needed locales.

utf8_general_ci and binary locales are built-in into Manticore. The first one is a generic collation for UTF-8 data (without any so-called language tailoring); it should behave similar to utf8_general_ci collation in MySQL. The second one is a simple bytewise comparison.

Collation can be overridden via SQL on a per-session basis using SET collation_connection statement. All subsequent SQL queries will use this collation. Otherwise all queries will use the server default collation or as specified in collation_server configuration directive. Manticore currently defaults to libc_ci collation.

Collations affect all string attribute comparisons, including those within ORDER BY and GROUP BY, so differently ordered or grouped results can be returned depending on the collation chosen. Note that collations don't affect full-text searching, for that use charset_table

Updating index schema

Updating index schema in RT mode

ALTER TABLE index {ADD|DROP} COLUMN column_name [{INTEGER|INT|BIGINT|FLOAT|BOOL|MULTI|MULTI64|JSON|STRING|TIMESTAMP}]

It supports adding one attribute at a time for both plain and RT indexes. The supported attribute types are:

  • int
  • bigint
  • float
  • bool
  • multi-valued
  • multi-valued 64bit
  • json
  • string

Important notes:

  • Querying an index is impossible (because of a write lock) while adding a column.
  • Newly created attribute's values are set to 0.
  • ALTER will not work for distributed indexes and indexes without any attributes.
  • DROP COLUMN will fail if an index has only one attribute.
Example
📋

mysql> desc rt;
+------------+-----------+
| Field      | Type      |
+------------+-----------+
| id         | bigint    |
| text       | field     |
| group_id   | uint      |
| date_added | timestamp |
+------------+-----------+
4 rows in set (0.01 sec)

mysql> alter table rt add column test integer;
Query OK, 0 rows affected (0.04 sec)

mysql> desc rt;
+------------+-----------+
| Field      | Type      |
+------------+-----------+
| id         | bigint    |
| text       | field     |
| group_id   | uint      |
| date_added | timestamp |
| test       | uint      |
+------------+-----------+
5 rows in set (0.00 sec)

mysql> alter table rt drop column group_id;
Query OK, 0 rows affected (0.01 sec)

mysql> desc rt;
+------------+-----------+
| Field      | Type      |
+------------+-----------+
| id         | bigint    |
| text       | field     |
| date_added | timestamp |
| test       | uint      |
+------------+-----------+
4 rows in set (0.00 sec)

Updating index FT settings in plain mode

ALTER RTINDEX index RECONFIGURE

ALTER can also reconfigure an RT index in plain mode, so that new tokenization, morphology, and other text processing settings from the configuration file take effect on the newly INSERT-ed rows, while retaining the existing rows as they were. Internally, it forcibly saves the current RAM chunk as a new disk chunk, and adjusts the index header, so that the new rows are tokenized using the new rules.

Example
📋
mysql> show index rt settings;
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| settings      |       |
+---------------+-------+
1 row in set (0.00 sec)

mysql> alter rtindex rt reconfigure;
Query OK, 0 rows affected (0.00 sec)

mysql> show index rt settings;
+---------------+----------------------+
| Variable_name | Value                |
+---------------+----------------------+
| settings      | morphology = stem_en |
+---------------+----------------------+
1 row in set (0.00 sec)