When searching for some keywords I was getting no results.
Attributes were searchable and I tried to raise StopWordThresholdValue and StopWordThresholdPercent to avoid stop words issues with no results.

Then I tried to rebuild index tables with bin/php/updatesearchindex.php and I was getting a SQL error, some chars were making the query syntax invaild.

This is the error from the log:

Query error (1366): Incorrect string value: '\xE9\x80' for column 'word' at row 134. Query: INSERT INTO
ezsearch_word ( word, object_count )
VALUES ('hat', '1' ), ('unternehmen', '1' ), ('seine', '1' ), ('kompetenz', '1' ), ('technisches', '1' ), ('genutzt', '1' ), ('realisieren', '1' ), ('welche', '1' ), ('dem', '1' ), ('hochsten', '1' ), ('qualitatsstandard', '1' ), ('gerecht', '1' ), ('werden', '1' ), ('an', '1' ), ('absoluten', '1' ), ('spitze', '1' ), ('bietet', '1' ), ('kuche', '1' ), ('aktuellem', '1' ), ('stil', '1' ), ('gestaltet', '1' ), ('tillverkat', '1' ), ('kåpor', '1' ), ('år', '1' ), ('trettio', '1' ), ('års', '1' ), ('erfarenhet', '1' ), ('och', '1' ), ('dagligt', '1' ), ('engagemang', '1' ), ('att', '1' ), ('kunna', '1' ), ('erbjuda', '1' ), ('dig', '1' ), ('uppmarksamhet', '1' ), ('detaljer', '1' ), ('basta', '1' ), ('europeisk', '1' ), ('under', '1' ), ('vilka', '1' ), ('utvecklat', '1' ), ('kunskap', '1' ), ('gjort', '1' ), ('mojligt', '1' ), ('oss', '1' ), ('skapa', '1' ), ('stor', '1' ), ('samling', '1' ), ('produkter', '1' ), ('med', '1' ), ('enastående', '1' ), ('personlighet', '1' ), ('teknik', '1' ), ('gått', '1' ), ('samman', '1' ), ('produkten', '1' ), ('gjorde', '1' ), ('falmecs', '1' ), ('medarbetare', '1' ), ('verkliga', '1' ), ('experter', '1' ), ('inom', '1' ), ('utveckling', '1' ), ('gor', '1' ), ('våra', '1' ), ('ett', '1' ), ('koncentrat', '1' ), ('spiskåpan', '1' ), ('excellens', '1' ), ('producem', '1' ), ('hote', '1' ), ('bucatarie', '1' ), ('peste', '1' ), ('treizeci', '1' ), ('ani', '1' ), ('experiența', '1' ), ('și', '1' ), ('angajament', '1' ), ('cotidian', '1' ), ('cu', '1' ), ('scopul', '1' ), ('va', '1' ), ('oferi', '1' ), ('calitate', '1' ), ('atenție', '1' ), ('detalii', '1' ), ('atingand', '1' ), ('cele', '1' ), ('mai', '1' ), ('inalte', '1' ), ('culmi', '1' ), ('ale', '1' ), ('excelenței', '1' ), ('europene', '1' ), ('care', '1' ), ('ne', '1' ), ('am', '1' ), ('dezvoltat', '1' ), ('competențele', '1' ), ('ce', '1' ), ('au', '1' ), ('permis', '1' ), ('cream', '1' ), ('colecție', '1' ), ('colosala', '1' ), ('produse', '1' ), ('personalitate', '1' ), ('remarcabila', '1' ), ('tehnologia', '1' ), ('pasiunea', '1' ), ('fuzionat', '1' ), ('pentru', '1' ), ('cel', '1' ), ('bun', '1' ), ('produs', '1' ), ('posibil', '1' ), ('oamenii', '1' ), ('devenit', '1' ), ('adevarați', '1' ), ('experți', '1' ), ('aspirații', '1' ), ('facand', '1' ), ('din', '1' ), ('produsele', '1' ), ('noastre', '1' ), ('simbol', '1' ), ('hotei', '1' ), ('抽油', '1' ), ('油烟', '1' ), ('烟机', '1' ), ('机', '1' ), ('专业', '1' ), ('业制', '1' ), ('制逧, '1' ), ('造三', '1' ), ('三十', '1' ), ('十年', '1' ), ('年', '1' ), ('年的', '1' ), ('的行', '1' ), ('行业', '1' ), ('业经', '1' ), ('经验', '1' ), ('验', '1' ), ('，', '1' ), ('以及', '1' ), ('及我', '1' ), ('我们', '1' ), ('们在', '1' ), ('在质', '1' ), ('质量', '1' ), ('量和', '1' ), ('和细', '1' ), ('细节', '1' ), ('节上', '1' ), ('上对', '1' ), ('对客', '1' ), ('客户', '1' ), ('户的', '1' ), ('的日', '1' ), ('日常', '1' ), ('常承', '1' ), ('承诺', '1' ), ('诺', '1' ), ('造就', '1' ), ('就了', '1' ), ('了今', '1' ), ('今日', '1' ), ('日能', '1' ), ('能在', '1' ), ('在欧', '1' ), ('欧洲', '1' ), ('洲享', '1' ), ('享誉', '1' ), ('誉最', '1' ), ('最佳', '1' ), ('佳。', '1' ), ('。三', '1' ), ('年间', '1' ), ('间', '1' ), ('们已', '1' ), ('已开', '1' ), ('开发', '1' ), ('发出', '1' ), ('出专', '1' ), ('专门', '1' ), ('门的', '1' ), ('的技', '1' ), ('技术', '1' ), ('术', '1' ), ('积累', '1' ), ('累了', '1' ), ('了经', '1' ), ('使我', '1' ), ('们能', '1' ), ('能创', '1' ), ('创逧, '1' ), ('造大', '1' ), ('大量', '1' ), ('量的', '1' ), ('的产', '1' ), ('产品', '1' ), ('品并', '1' ), ('并获', '1' ), ('获得', '1' ), ('得绝', '1' ), ('绝佳', '1' ), ('佳声', '1' ), ('声誉', '1' ), ('誉。', '1' ), ('。', '1' ), ('历经', '1' ), ('经', '1' ), ('年技', '1' ), ('术和', '1' ), ('和激', '1' ), ('激情', '1' ), ('情的', '1' ), ('的完', '1' ), ('完美', '1' ), ('美融', '1' ), ('融合', '1' ), ('合', '1' ), ('为您', '1' ), ('您提', '1' ), ('提供', '1' ), ('供最', '1' ), ('最优', '1' ), ('优质', '1' ), ('质的', '1' ), ('品', '1' ), ('；', '1' ), ('这', '1' ), ('年使', '1' ), ('使得', '1' ), ('得', '1' ), ('fammec', '1' ), ('人成', '1' ), ('成为', '1' ), ('为真', '1' ), ('真正', '1' ), ('正的', '1' ), ('的专', '1' ), ('专家', '1' ), ('家', '1' ), ('也让', '1' ), ('让我', '1' ), ('们的', '1' ), ('品优', '1' ), ('优秀', '1' ), ('秀地', '1' ), ('地成', '1' ), ('为抽', '1' ), ('机的', '1' ), ('的象', '1' ), ('象征', '1' ), ('征。', '1' )

There are some chinese chars that are added to the query but result without the string delimiter.

I tried to fix the issue by editing addObject() in ezsearchengine.php by utf8-encoding the result of this line:

$text = eZSearchEngine::normalizeText( htmlspecialchars ($metaDataPart['text'], ENT_NOQUOTES, 'UTF-8' ) , true );

which becomes:

$text = utf8_encode(eZSearchEngine::normalizeText( htmlspecialchars ($metaDataPart['text'], ENT_NOQUOTES, 'UTF-8' ) , true ));

After that it seems working properly