Elasticsearch: should + minimum_should_match vs must - elasticsearch

Question

I test with these 2 queries
Query with must
{
"size": 200,
"from": 0,
"query": {
"bool": {
"must": [ {
"match": {
"_all": "science"
}
},
{
"match": {
"category": "fiction"
}
},
{
"match": {
"country": "us"
}
}
]
}
}
}
Query with should + minimum_should_match
{
"size": 200,
"from": 0,
"query": {
"bool": {
"should": [ {
"match": {
"_all": "science"
}
},
{
"match": {
"category": "fiction"
}
},
{
"match": {
"country": "us"
}
}
],
minimum_should_match: 3
}
}
}
Both queries give me same result, I don't know the difference between these 2, when we should use minimum_should_match?

Answer 1

I guess you mean minimum_number_should_match, right?
In both cases it would be the same because you have the same number of clauses in should. minimum_number_should_match usually is used when you have more clauses than the number you specify there.
For example if you have 5 should clauses, but for some reason you only need three of them to be fulfilled you would do something like this:
{
"query": {
"bool": {
"should": [
{
"term": {
"tag": "wow"
}
},
{
"term": {
"tag": "elasticsearch"
}
},
{
"term": {
"tag": "tech"
}
},
{
"term": {
"user": "plchia"
}
},
{
"range": {
"age": {
"gte": 10,
"lte": 20
}
}
}
],
"minimum_should_match": 3
}
}
}

Answer 2

That's correct and desired behavior. Let's decipher it a little bit:
Boolean query with must clauses means that all clauses under must section are required to match. Just like in English - it means strong obligation.
Boolean query with should clauses means that some clauses are required to match, whereas the others are not (i.e. soft obligation). The default number of clauses that must match here is simply 1. And to override this behavior the minimum_should_match parameter is coming into play. If you specify minimum_should_match=3 it will mean 3 clauses under should must match. From the practical perspective it exactly the same as specifying those clauses with must.
Hope it explains it in details.