Message Searching Options 
Path to Message Searching Options: Settings > Options > Message Searching Options

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Search Engine Enabled (Yes/No)
Whether or not to allow searching for posts and threads within the channels.

This is a server-intensive process so you might want to disable it if you start to have performance problems.

Queue Search Updates (Yes/No)
Whether or not to queue updates for search.

When this option is set to Yes, changes to searchable content is not immediately reflected in search results. This can improve the response time for users making edits on larger sites.

Minimum Time Between Searches (default: 0)
Minimum time (in seconds) that must expire before the user can do a new search.

Set this to 0 to allow users to search as frequently as they want.

Search Results Posts Per Page (default: 25)
Number of successful search results to display per page.

This number must be at least 1.

Maximum Search Results to Return (default: 500)
Maximum results to be returned.

Any results over this amount are discarded.

Search Index Minimum Word Length (default: 4)
Minimum string length to be searched.

The smaller this number is, the larger the site search index and database will be.

Search Index Maximum Word Length (default: 20)
Maximum string length to be searched.

For example, if this is set to 15, users cannot search for 16-letter words.

When using full-text search, this option limits the size of words that can be searched. Smaller words take longer to search because they are more common. MySQL Fulltext has its own minimum word length as well, that must be changed at the server level.

Words to be excluded from search
If there are special words that are very common for your site, this option prevents their being searched.

Separate each word to be excluded with a space.

Searching for very common words on a large site can be server intensive.
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