Struct FieldMetadata 

pub struct FieldMetadata { /* private fields */ }

Literal metadata

Stores metadata associated with a literal expressions and is designed to be fast to clone.

This structure is used to store metadata associated with a literal expression, and it corresponds to the metadata field on Field.

Example: Create FieldMetadata from a Field

// Create a new `FieldMetadata` instance from a `Field`
let metadata = FieldMetadata::new_from_field(&field);
// There is also a `From` impl:
let metadata = FieldMetadata::from(&field);

Example: Update a Field with FieldMetadata

// Add any metadata from `FieldMetadata` to `Field`
let updated_field = metadata.add_to_field(field);

Implementations

impl FieldMetadata

pub fn new_empty() -> Self

Create a new empty metadata instance.

pub fn merge_options( m: Option<&FieldMetadata>, n: Option<&FieldMetadata>, ) -> Option<FieldMetadata>

Merges two optional FieldMetadata instances, overwriting any existing keys in m with keys from n if present.

This function is commonly used in alias operations, particularly for literals with metadata. When creating an alias expression, the metadata from the original expression (such as a literal) is combined with any metadata specified on the alias.

Arguments

• m - The first metadata (typically from the original expression like a literal)
• n - The second metadata (typically from the alias definition)

Merge Strategy

• If both metadata instances exist, they are merged with n taking precedence
• Keys from n will overwrite keys from m if they have the same name
• If only one metadata instance exists, it is returned unchanged
• If neither exists, None is returned

Example usage

use datafusion_expr::expr::FieldMetadata;
use std::collections::BTreeMap;

// Create metadata for a literal expression
let literal_metadata = Some(FieldMetadata::from(BTreeMap::from([
    ("source".to_string(), "constant".to_string()),
    ("type".to_string(), "int".to_string()),
])));

// Create metadata for an alias
let alias_metadata = Some(FieldMetadata::from(BTreeMap::from([
    ("description".to_string(), "answer".to_string()),
    ("source".to_string(), "user".to_string()), // This will override literal's "source"
])));

// Merge the metadata
let merged = FieldMetadata::merge_options(
    literal_metadata.as_ref(),
    alias_metadata.as_ref(),
);

// Result contains: {"source": "user", "type": "int", "description": "answer"}
assert!(merged.is_some());

pub fn new_from_field(field: &Field) -> Self

Create a new metadata instance from a Field’s metadata.

pub fn new(inner: BTreeMap<String, String>) -> Self

Create a new metadata instance from a map of string keys to string values.

pub fn inner(&self) -> &BTreeMap<String, String>

Get the inner metadata as a reference to a BTreeMap.

pub fn into_inner(self) -> Arc<BTreeMap<String, String>>

Return the inner metadata

pub fn extend(&mut self, other: Self)

Adds metadata from other into self, overwriting any existing keys.

pub fn is_empty(&self) -> bool

Returns true if the metadata is empty.

pub fn len(&self) -> usize

Returns the number of key-value pairs in the metadata.

pub fn to_hashmap(&self) -> HashMap<String, String>

Convert this FieldMetadata into a HashMap<String, String>

pub fn add_to_field(&self, field: Field) -> Field

Updates the metadata on the Field with this metadata, if it is not empty.

Trait Implementations

impl Clone for FieldMetadata

fn clone(&self) -> FieldMetadata

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for FieldMetadata

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for FieldMetadata

fn default() -> Self

Returns the “default value” for a type.

impl From<&Field> for FieldMetadata

fn from(field: &Field) -> Self

Converts to this type from the input type.

impl From<&HashMap<String, String>> for FieldMetadata

From reference

fn from(map: &HashMap<String, String>) -> Self

Converts to this type from the input type.

impl From<&HashMap<String, String>> for FieldMetadata

fn from(map: &HashMap<String, String>) -> Self

Converts to this type from the input type.

impl From<BTreeMap<String, String>> for FieldMetadata

fn from(inner: BTreeMap<String, String>) -> Self

Converts to this type from the input type.

impl From<HashMap<String, String>> for FieldMetadata

fn from(map: HashMap<String, String>) -> Self

Converts to this type from the input type.

impl From<HashMap<String, String>> for FieldMetadata

From hashbrown map

fn from(map: HashMap<String, String>) -> Self

Converts to this type from the input type.

impl Hash for FieldMetadata

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl PartialEq for FieldMetadata

fn eq(&self, other: &FieldMetadata) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl PartialOrd for FieldMetadata

fn partial_cmp(&self, other: &FieldMetadata) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl Eq for FieldMetadata

impl StructuralPartialEq for FieldMetadata