Relationship of the word (index) table
+ qa_titlewords *2
+ qa_contentwords *3
+ qa_posttags *4
+ qa_tagwords *5
Maintenance of the word table with phpMyAdmin
*1 table has foreign key constraint. Therefore, if you want to maintain the word table (*1) by phpMyAdmin, you need to truncate tables under the order.
*2 > *3 > *4 > *5 > *1
Maintenance of the word table with "Stats" option of Q2A
"Admin" > "Stats" > "Reindex content"
Tables from *1 to *5 will be refleshed. However, there is one important point. Fragmentation may occur on your disk by this process. As a result, your disk may not be reduced. In that case, your database (disk) may be reduced by optimizing tables by phpMyAdmin.
These operations are dangerous. I recommend that you try these operation after backing up your database. Of course, try out at your own risk.