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Multiword synomyms in SolR
Medical domain has multiple ways to tell about the same thing. It becomes handy to use and maintain muliword synonyms thesauri.
SolR recently released a built-in solution to handle multi-word synonyms and I have tested it successfully:
- synonyms are computed at query time only.
- using multi-word synonyms are only compatible with simple/lucene/eDismax query parsers right now. complexphrase is broken with such dictionary.
- multi-word synonyms are supposed to be used before stemming
- adding new synonyms needs to delete the collection and reindex it. (to be confirmed)
Some questions remains: 1) How many synonyms can solr handle and still perform well ? 2) How to make synonyms addition/modifications without rebooting ? 3) Which is the better query parser together with multi-word synonyms ?
The simple query parser
Given this field: text:"say me bye or say hello world" And the given configuration:
- one word text:(hello) matches "say me bye or say <em>hello</em> world"
- one regex text:(/hel./)** matches "say me bye or say <em>hello</em> world"
- not one word text:(hello) matches "say me bye or say <em>hello</em> world"
- one tailing wildcard text:(o)** matches "say me bye or say <em>hello</em> world"
- one fuzzy word text:(word~) matches "say me bye or say hello <em>world</em>"
- two words text:(hello AND bye) matches "say me <em>bye</em> or say <em>hello</em> world"
- two words exact text:("hello world") matches "say me bye or say <em>hello</em> <em>world</em>"
- two words with distance text:("say world"~2) matches "say me bye or <em>say</em> hello <em>world</em>"
- one word AND two words with distance text:(bye AND "hello world"~2) matches "say me <em>bye</em> or say <em>hello</em> <em>world</em>"
- two words with distance AND two words with distance text:("say bye"~2 AND "say world"~2) matches "<em>say</em> me <em>bye</em> or <em>say</em> hello <em>world</em>"
- one multi-word synonym: text:("see you soon"~2 AND "hello world"~2) matches "say me <em>bye</em> or say <em>hello</em> <em>world</em>"
Note: The simple query parser has some limations compared to the complexphrase parser:
- it only keeps order for exact phrases
- fuzzy, joker are not allowed for phrases
However it has some advantages:
- it allows to define multiple groups of words
- it allows regex on one word
How to handle negation
SolR allows to look for the absence of a word or phrase, but not for negations of them. One solution I found is to exploit the multi-word synonyms, by linking all the various ways to express negation to the neg word. Then it is possible to ask for the absence of a negation.
This can be used in this way:
- "text": "I never had any trouble"
- searching for people with trouble: text:((trouble) AND NOT ("neg trouble"))
Generalisation of negation
For a given simple full-text low level filter such a word or a phrase:
- simple => (simple AND NOT "neg simple")
- "very simple" => ("very simple" AND NOT "neg very simple)
- less simple => ( (less AND NOT "neg less") OR (simple AND NOT "neg simple) )
- less AND simple => ( (less AND NOT "neg less") AND (simple AND NOT "neg simple) )
Simplifying interface for user
The more user friendly interface in my mind would allo people to write group of words and articulate them with logical operator such AND/OR/NOT. You don't want them to manage phrase and word too. So when a user look for:
- see you soon => "see you soon"
They might check for ordered and compacted word, but by default we the tolerate some distance:
- see you soon => "see you soon" => "see you soon"~4
In order to remove negated pattern, we apply the above method:
- see you soon => "see you soon" => "see you soon"~4 => "see you soon"~4 AND NOT "neg see you soon"~4
People might add other groups and articulate them together:
- ("see you soon"~4 AND NOT "neg see you soon"~4) NOT ("an other example"~4 AND NOT "neg an other example"~4)
The above example would match a text containing see you soon and not containing an other example. By the way any multi-word synonym would be translated. This also means the user not able to use jocker or fuzzy search. However, the jocker can be replaced with the stemming process, which has the advantage of not breaking the performances in case of very narrow joker query and also being transparent for the end user. There is also a possible replacement for fuzzy feature transparent to user.
Fuzzy search with synonyms
Word Embedding offers the opportunity to freely produce typo synonyms. I have tested succesfully a quite large list of them (~200k entries) and the performances where not impacted. So most common error are transparently integrated to end users.
Dated, Delay and other structured informations
The simple query parser let add some structured informations within the free-text to be queried. For example the below enriched text allows:
- "text":"delay0002 dateCreat20181214 my example free-text"
- text:([delay0001 TO delay0003] AND [dateCreat201812 TO NOW] AND "example") this makes possible to filter based on dates.
It is also possible to add many coded informations within the text to be queried as structured data with full-text structured capabilities. This will be very useful to get NLP pipelines results.
Dealing with multivalued fields (MV)
While the MVs look temptating for storing multiple occurence of the same concept, they are not a good choice when dealing with full text queries. Indeed the multiple values are not considered as independant values but as a whole with some defined distance between values.
So how to modelize multiple occurences of the same concept within SolR ? One alternate solution is to use multiple single fields for textual datatype. For example it the encounter has two physician notes both with a adverse event section, the resulting encounter document will have two fields "pn.adverse1" and "pn.averse2". This implies that when the user asks for something present into this section, the resulting query should be modified accordingly
- "pn.adverse1":"hello world"
- "pn.adverse2":"my example"
- "pn.adverse:(hello AND example)" will be replaced by:
- "pn.adverse1:(hello AND example) OR pn.adverse2:(hello AND example)"
This makes also possible to mix full text queries with structured queries based on the same field. Also there is some drawbacks. The replacement mecanism makes mandatory to know in advance the maximum number of fields of every documents. An other drawback is when the user wants to look into every section of the same document. One solution is to copyField
The final method offers:
- multi-word synonyms
- monoword synonyms
- monoword typos (fuzzy match)
- AND/OR/NOT operators
Still some aspects are missing:
- no weighted queries (but not needed for my use case)
- no multivalued textual fields (this parser does not make difference between them)
This lets envisage a simple but powerful interface. Let's now see how to transform a medical relational database and populate SolR.