The built-in WordPress search matches letters, not words. A visitor types “светильники” (lamps) and a page containing “светильник” (lamp) is not found. They type “кровати” (beds) and the product “кровать” (bed) is not found. The visitor concludes you do not sell it and leaves.
Poiskovich reduces words to their stem, so every form of a word is searched as one word.
Built-in WordPress search matches words literally, so a query for a plural or an inflected form misses posts that use another form of the same word. Poiskovich reduces both the query and the indexed text to word stems, so “светильники” finds “светильник”.
Stemming is enabled out of the box and needs no configuration.
The plugin builds its own index: posts are split into words in advance instead of being scanned on every visitor request. A LIKE search cannot use database indexes, and on a catalogue of a few thousand products that is noticeable even on good hosting.
The index is compact — one row per “word + post” pair. New and edited posts are indexed automatically; a full rebuild is only needed after changing the settings or after a bulk content edit, and it runs in batches so it does not hit the PHP time limit.
Nothing has to be changed in your theme: the plugin replaces the results of the standard WordPress search, so your theme template, pagination and everything else keep working.
The plugin sends nothing anywhere. It makes no external requests and stores all data in your own database. The query log records the search text, the number of results and the date — no IP addresses and no visitor identifiers.