Expression system

Introduction

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 example creates a reference for the student field.

JSON

Create a field reference

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

Java

Create a field reference

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

Literal

Literal is a constant value. The following example creates a NULL literal and literals of three different data types for the value 1024.

JSON

Create literal expressions

[
  {
    "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

Create literal expressions

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 example creates function expressions for rand() and date_trunc('year', birthday).

JSON

Create function expressions

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

Java

Create function expressions

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, 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

Read an unparsed expression

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

Java

Read an unparsed expression

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