Expression system of Apache Gravitino

This page introduces the expression system of Apache Gravitino. Expressions are vital component of metadata definition, through expressions, you can define default values for columns, function arguments for function partitioning, bucketing, and sort term of sort ordering in tables. Gravitino expression system divides expressions into three basic parts: field reference, literal, and function. Function expressions can contain field references, literals, and other function expressions.

Field reference

Field reference is a reference to a field in a table. The following is an example of creating a field reference expression, demonstrating how to create a reference for the student field.

Json

[
  {
    "type": "field",
    "fieldName": [
      "student"
    ]
  }
]

Java

NamedReference field = NamedReference.field("student");

Literal

Literal is a constant value. The following is an example of creating a literal expression, demonstrating how to create a NULL literal and three different data types of literal expressions for the value 1024.

Json

[
  {
    "type": "literal",
    "dataType": "null",
    "value": "null"
  },
  {
    "type": "literal",
    "dataType": "integer",
    "value": "1024"
  },
  {
    "type": "literal",
    "dataType": "string",
    "value": "1024"
  },
  {
    "type": "literal",
    "dataType": "decimal(10,2)",
    "value": "1024"
  }
]

Java

Literal<?>[] literals =
    new Literal[] {
    Literals.NULL,
    Literals.integerLiteral(1024),
    Literals.stringLiteral("1024"),
    Literals.decimalLiteral(Decimal.of("1024", 10, 2))
    };

Function expression

Function expression represents a function call with/without arguments. The arguments can be field references, literals, or other function expressions. The following is an example of creating a function expression, demonstrating how to create function expressions for rand() and date_trunc('year', birthday).

Json

[
  {
    "type": "function",
    "funcName": "rand",
    "funcArgs": []
  },
  {
    "type": "function",
    "funcName": "date_trunc",
    "funcArgs": [
      {
        "type": "literal",
        "dataType": "string",
        "value": "year"
      },
      {
        "type": "field",
        "fieldName": [
          "birthday"
        ]
      }
    ]
  }
]

Java

FunctionExpression[] functionExpressions =
        new FunctionExpression[] {
          FunctionExpression.of("rand"),
          FunctionExpression.of("date_trunc", Literals.stringLiteral("year"), NamedReference.field("birthday"))
        };

Unparsed expression

Unparsed expression is a special type of expression, currently serves exclusively for presenting the default value of a column when it's unsolvable. The following shows the data structure of an unparsed expression in JSON and Java, enabling easy retrieval of its value.

Json

{
  "type": "unparsed",
  "unparsedExpression": "(curdate() + interval 1 year)"
}

Java

// The result of the following expression is a string "(curdate() + interval 1 year)"
String unparsedValue = ((UnparsedExpression) expressino).unparsedExpression();