What is Full Text Search vs LIKE

Question

In general, there is a tradeoff between "precision" and "recall". High precision means that fewer irrelevant results are presented (no false positives), while high recall means that fewer relevant results are missing (no false negatives). Using the LIKE operator gives you 100% precision with no concessions for recall. A full text search facility gives you a lot of flexibility to tune down the precision for better recall.

Most full text search implementations use an "inverted index". This is an index where the keys are individual terms, and the associated values are sets of records that contain the term. Full text search is optimized to compute the intersection, union, etc. of these record sets, and usually provides a ranking algorithm to quantify how strongly a given record matches search keywords.

The SQL LIKE operator can be extremely inefficient. If you apply it to an un-indexed column, a full scan will be used to find matches (just like any query on an un-indexed field). If the column is indexed, matching can be performed against index keys, but with far less efficiency than most index lookups. In the worst case, the LIKE pattern will have leading wildcards that require every index key to be examined. In contrast, many information retrieval systems can enable support for leading wildcards by pre-compiling suffix trees in selected fields.

Other features typical of full-text search are

• lexical analysis or tokenization—breaking a block of unstructured text into individual words, phrases, and special tokens
• morphological analysis, or stemming—collapsing variations of a given word into one index term; for example, treating "mice" and "mouse", or "electrification" and "electric" as the same word
• ranking—measuring the similarity of a matching record to the query string

FTS involves indexing the individual words within a text field in order to make searching through many records quick. Using LIKE still requires you to do a string search (linear or the like) within the field.

MySQL creates an index from the words of the enabled full-text search column and performs searches on this index. MySQL uses a sophisticated algorithm to determine the rows matched against the search query.

Also, from this SO answer:

There's a few advantages to full text searching.

Indexing:

Something like:

WHERE Foo LIKE '%Bar';

Cannot take advantage of an index. It has to look at every single row, and see if it matches. A fulltext index, however, can. In fact, fulltext indexes can offer a lot more flexibility in terms of the order of matching words, how close those words are together, etc.

Stemming:

A fulltext search can stem words. If you search for run, you can get results for "ran" or "running". Most fulltext engines have stem dictionaries in a variety of languages.

Weighted Results:

A fulltext index can encompass multiple columns. For example, you can search for "peach pie", and the index can include a title, keywords, and a body. Results that match the title can be weighted higher, as more relevant, and can be sorted to show near the top.

Disadvantages:

A fulltext index can potentially be huge, many times larger than a standard B-TREE index. For this reason, many hosted providers who offer database instances disable this feature, or at least charge extra for it. For example, last I checked, Windows Azure did not support fulltext queries.

Fulltext indexes can also be slower to update. If the data changes a lot, there might be some lag updating indexes compared to standard indexes.

Like uses wildcards only, and isn't all that powerful.

Full text allows much more complex searching, including And, Or, Not, even similar sounding results (SOUNDEX) and many more items.

I would start looking at the SQL CONTAINS() FREETEXT() and related Full Text search items to help get a better understanding of what is available.