How much disk overhead will indexing require?
The overhead, the amount of space needed for the index tables, varies between about 50% of the original text volume and 200%. Generally, the larger the total amount of text, the smaller the overhead, but many small records will use more overhead than fewer large records. Also, clean data (such as published text) will require less overhead than dirty data such as emails or discussion notes, since the dirty data is likely to include many unique words from mis-spellings and abbreviations. A text-only index is smaller than a combined text and theme index. A prefix and substring index makes the index significantly larger.
