Was hard to diagnose...
After upgrading a Tiki20 to Tiki21 I was able to rebuilt the index:
[03-03-2020 12:05] Started rebuilding index... logging to file: /var/tmp/Search_Indexer_mysql_tsaharoniki_tiki21_console.log Unified search engine: MySQL, version 5.7.26 error: The search index could not be rebuilt. Expression #1 of ORDER BY clause is not in SELECT list, references column 'tsaharoniki_tiki21.ttif0.value' which is not in SELECT list; this is incompatible with DISTINCT
2020-03-03T10:47:35+02:00 ERR (3): WARNING: PDOStatement::execute(): SQLSTATE[HY000]: General error: 3065 Expression #1 of ORDER BY clause is not in SELECT list, references column 'tsaharoniki_tiki21.tti$
Checking the item (I had to find it!) I found out that having a sort parameter in an itemList field was responsible.
Once removed it worked.
This is very problematic when you have financial data as any report must be ordered (by date).
Same same in Tiki 20
It may be related to the removal of the "split" function in PHP 7+;
It is now also on Tiki18.8 as it was signaled to me by another Tiki user (tested with him)
I tried to reproduce on the show instance but there is a bug with field list on the trackers and it is not possible to recreate the caches.
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