[Joins] Join Query DSL

[harshavamsi]

Is your feature request related to a problem? Please describe

Coming from #15185 , we want to introduce the join DSL format that will be used to construct the join query. It will make use of the existing QueryBuilders within OpenSearch to parse the left and right queries. We will add new logic to SearchSourceBuilder to support the new join field in the query DSL.

Describe the solution you'd like

The join field will be parsed by a new JoinBuilder in OpenSearch that will take in the following:

• left query(just query from the builder perspective) from StreamInput and parses into Lucene query,
• fields - left query fields to broadcast back
• join - XContent Object containing
  • right query
    • index - right index to perform join
    • query - right lucene query
    • fields - right query fields to join on
    • type - type of join to perform(inner, outer, cross, left_join, right_join)
    • algorithm - type of join algorithm to use(hash_join, nested_join), we might support only one type to start with
    • condition - the join condition to evaluate while joining
      • left_field - left index field to use while evaluating
      • right_field - right index field to use while evaluating
      • comparator - the operator to use for the condition(<, <=, >, >=, =) // should this be just text?
    • fields - right query fields to broadcast back
    • aggs - aggregations to perform while joining / should this be outside the join clause?

Full query DSL

{  
  "query": {  
    "bool": {  
      "filter": [  
        {  
          "range": {  
            "@timestamp": {  
              "gte": "now-1h"  
            }  
          }  
        },  
        {  
          "match": {  
            "message": "error"  
          }  
        }  
      ]  
    }  
  },  
  "fields": ["instance_id", "status_code"],  
  "join": {  
    "right_query": {  
        "index": "instance_details",   
        "query": {  
          "range": {  
            "created_at": {  
              "gte": "now-1y"  
            }  
          }  
        },  
        "fields": ["instance_id", "region"]  
    },  
    "type": "inner",   
    "algorithm": "hash_join", // optional  
    "condition": {  
        "left_field": "instance_id",  
        "right_field": "instance_id",  
        "comparator": "="  
    },  
    "fields": ["region", "status_code"],  
    "aggs": {  
      "by_region": {  
        "terms": {  
          "field": "region"  
        },  
        "aggs": {  
          "by_status_code": {  
            "terms": {  
              "field": "status_code"  
            },  
            "aggs": {  
              "status_code_count": {  
                "value_count": {  
                  "field": "status_code"  
                }  
              }  
            }  
          }  
        }  
      }  
    }  
  }  
}

Related component

Search:Query Capabilities

Describe alternatives you've considered

No response

Additional context

No response

[smacrakis]

Small comment: why do we speak of the left and right queries? In SQL, the left and right objects are normally called "tables". The result sets to join may be defined by table names or by subqueries. The usual equivalent of "table" in OpenSearch is "index", but that term is so overloaded that it's best avoided. Wouldn't it be clearest for people who are familiar with SQL to use the standard SQL terminology, namely tables?