1.  DNN daily cleanning:

Go to host>SQL (run as script)

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truncate table {databaseOwner}{objectQualifier}SearchItemWordPosition

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2. Reindex dnn site's search:

First clean up the tables: host>SQL>run as script:

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delete {databaseOwner}{objectQualifier}SearchItemWordPosition
delete {databaseOwner}{objectQualifier}SearchItemWord
delete {databaseOwner}{objectQualifier}SearchWord
delete {databaseOwner}{objectQualifier}SearchItem

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Then re-index the content:

host>search admin>re-index the content