Phrase search queries are generally more expensive to process than normal conjunctive search queries.

In addition to the work associated with a conjunctive query, a phrase search operation must verify the presence of the exact requested phrase.

The cost of phrase search operations depends mostly on how frequently the query words appear in the data. Searches for phrases containing relatively infrequent words (such as proper names) are generally very rapid, because the base conjunctive search narrows the results to a small set of candidate hits, and within these hits relatively few possible match positions need to be considered.

On the other hand, searches for phrases containing only very common words are more expensive. For example, consider a search for the phrase "to be or not to be" on a large collection of documents. Because all of these words are quite common, the base conjunctive search does not narrow the set of candidate hit documents significantly. Then, within each candidate result document, numerous possible word positions need to be scanned, because these words tend to be frequently reused within a single document.

Even very difficult queries (such as "to be or not to be") are handled by the MDEX Engine within a few seconds (depending on hardware), and possibly faster on moderate sized data sets. Obviously, if such queries are expected to be very common, adequate hardware must be employed to ensure sufficient throughput. In most applications, phrase searches tend to be used far less frequently than normal searches. Also, most phrase searches performed tend to contain at least one information-rich, low-frequency word, enabling results to be returned rapidly (that is, in less than a second).

You can use the --phrase_max <num> flag for the dgraph to specify the maximum number of words in each phrase for text search. Using this flag improves performance of text search with phrases. The default number is 10. If the maximum number of words in a phrase is exceeded, the phrase is truncated to the maximum word count and a warning is logged.