How much disk overhead will indexing require?

Answer: The overhead, the amount of space needed for the index tables, varies between about 50 and 200 percent of the original text volume. Generally, larger amounts of text result in smaller overhead, but many small records use more overhead than fewer large records. Also, clean data (such as published text) requires less overhead than dirty data such as emails or discussion notes, because the dirty data is likely to include many misspelled and abbreviated words.

A text-only index is smaller than a combined text and theme index. A prefix and substring index makes the index significantly larger.