9.15. JSON Functions and Operators
Table 9.40 shows the operators that are available for use with the two JSON data types (see Section 8.14).
Table 9.40. json and jsonb Operators
| Operator | Right Operand Type | Description | Example | Example Result |
|---|---|---|---|---|
-> | int | Get JSON array element (indexed from zero, negative integers count from the end) | '[{"a":"foo"},{"b":"bar"},{"c":"baz"}]'::json->2 | {"c":"baz"} |
-> | text | Get JSON object field by key | '{"a": {"b":"foo"}}'::json->'a' | {"b":"foo"} |
->> | int | Get JSON array element as text | '[1,2,3]'::json->>2 | 3 |
->> | text | Get JSON object field as text | '{"a":1,"b":2}'::json->>'b' | 2 |
#> | text[] | Get JSON object at specified path | '{"a": {"b":{"c": "foo"}}}'::json#>'{a,b}' | {"c": "foo"} |
#>> | text[] | Get JSON object at specified path as text | '{"a":[1,2,3],"b":[4,5,6]}'::json#>>'{a,2}' | 3 |
Note
There are parallel variants of these operators for both the json and jsonb types. The field/element/path extraction operators return the same type as their left-hand input (either json or jsonb), except for those specified as returning text, which coerce the value to text. The field/element/path extraction operators return NULL, rather than failing, if the JSON input does not have the right structure to match the request; for example if no such element exists. The field/element/path extraction operators that accept integer JSON array subscripts all support negative subscripting from the end of arrays.
The standard comparison operators shown in Table 9.1 are available for jsonb, but not for json. They follow the ordering rules for B-tree operations outlined at Section 8.14.4.
Some further operators also exist only for jsonb, as shown in Table 9.41. Many of these operators can be indexed by jsonb operator classes. For a full description of jsonb containment and existence semantics, see Section 8.14.3. Section 8.14.4 describes how these operators can be used to effectively index jsonb data.
Table 9.41. Additional jsonb Operators
| Operator | Right Operand Type | Description | Example |
|---|---|---|---|
@> | jsonb | Does the left JSON value contain the right JSON path/value entries at the top level? | '{"a":1, "b":2}'::jsonb @> '{"b":2}'::jsonb |
<@ | jsonb | Are the left JSON path/value entries contained at the top level within the right JSON value? | '{"b":2}'::jsonb <@ '{"a":1, "b":2}'::jsonb |
? | text | Does the string exist as a top-level key within the JSON value? | '{"a":1, "b":2}'::jsonb ? 'b' |
?| | text[] | Do any of these array strings exist as top-level keys? | '{"a":1, "b":2, "c":3}'::jsonb ?| array['b', 'c'] |
?& | text[] | Do all of these array strings exist as top-level keys? | '["a", "b"]'::jsonb ?& array['a', 'b'] |
|| | jsonb | Concatenate two jsonb values into a new jsonb value | '["a", "b"]'::jsonb || '["c", "d"]'::jsonb |
- | text | Delete key/value pair or string element from left operand. Key/value pairs are matched based on their key value. | '{"a": "b"}'::jsonb - 'a' |
- | integer | Delete the array element with specified index (Negative integers count from the end). Throws an error if top level container is not an array. | '["a", "b"]'::jsonb - 1 |
#- | text[] | Delete the field or element with specified path (for JSON arrays, negative integers count from the end) | '["a", {"b":1}]'::jsonb #- '{1,b}' |
Note
The || operator concatenates the elements at the top level of each of its operands. It does not operate recursively. For example, if both operands are objects with a common key field name, the value of the field in the result will just be the value from the right hand operand.
Table 9.42 shows the functions that are available for creating json and jsonb values. (There are no equivalent functions for jsonb, of the row_to_json and array_to_json functions. However, the to_jsonb function supplies much the same functionality as these functions would.)
Table 9.42. JSON Creation Functions
| Function | Description | Example | Example Result |
|---|---|---|---|
| Returns the value as json or jsonb. Arrays and composites are converted (recursively) to arrays and objects; otherwise, if there is a cast from the type to json, the cast function will be used to perform the conversion; otherwise, a scalar value is produced. For any scalar type other than a number, a Boolean, or a null value, the text representation will be used, in such a fashion that it is a valid json or jsonb value. | to_json('Fred said "Hi."'::text) | "Fred said \"Hi.\"" |
array_to_json(anyarray [, pretty_bool]) | Returns the array as a JSON array. A Postgres Pro multidimensional array becomes a JSON array of arrays. Line feeds will be added between dimension-1 elements if pretty_bool is true. | array_to_json('{{1,5},{99,100}}'::int[]) | [[1,5],[99,100]] |
row_to_json(record [, pretty_bool]) | Returns the row as a JSON object. Line feeds will be added between level-1 elements if pretty_bool is true. | row_to_json(row(1,'foo')) | {"f1":1,"f2":"foo"} |
| Builds a possibly-heterogeneously-typed JSON array out of a variadic argument list. | json_build_array(1,2,'3',4,5) | [1, 2, "3", 4, 5] |
| Builds a JSON object out of a variadic argument list. By convention, the argument list consists of alternating keys and values. | json_build_object('foo',1,'bar',2) | {"foo": 1, "bar": 2} |
| Builds a JSON object out of a text array. The array must have either exactly one dimension with an even number of members, in which case they are taken as alternating key/value pairs, or two dimensions such that each inner array has exactly two elements, which are taken as a key/value pair. |
| {"a": "1", "b": "def", "c": "3.5"} |
| This form of json_object takes keys and values pairwise from two separate arrays. In all other respects it is identical to the one-argument form. | json_object('{a, b}', '{1,2}') | {"a": "1", "b": "2"} |
Note
array_to_json and row_to_json have the same behavior as to_json except for offering a pretty-printing option. The behavior described for to_json likewise applies to each individual value converted by the other JSON creation functions.
Note
The hstore extension has a cast from hstore to json, so that hstore values converted via the JSON creation functions will be represented as JSON objects, not as primitive string values.
Table 9.43 shows the functions that are available for processing json and jsonb values.
Table 9.43. JSON Processing Functions
| Function | Return Type | Description | Example | Example Result |
|---|---|---|---|---|
| int | Returns the number of elements in the outermost JSON array. | json_array_length('[1,2,3,{"f1":1,"f2":[5,6]},4]') | 5 |
|
| Expands the outermost JSON object into a set of key/value pairs. | select * from json_each('{"a":"foo", "b":"bar"}') | key | value -----+------- a | "foo" b | "bar" |
| setof key text, value text | Expands the outermost JSON object into a set of key/value pairs. The returned values will be of type text. | select * from json_each_text('{"a":"foo", "b":"bar"}') | key | value -----+------- a | foo b | bar |
|
| Returns JSON value pointed to by path_elems (equivalent to #> operator). | json_extract_path('{"f2":{"f3":1},"f4":{"f5":99,"f6":"foo"}}','f4') | {"f5":99,"f6":"foo"} |
| text | Returns JSON value pointed to by path_elems as text (equivalent to #>> operator). | json_extract_path_text('{"f2":{"f3":1},"f4":{"f5":99,"f6":"foo"}}','f4', 'f6') | foo |
| setof text | Returns set of keys in the outermost JSON object. | json_object_keys('{"f1":"abc","f2":{"f3":"a", "f4":"b"}}') | json_object_keys ------------------ f1 f2 |
| anyelement | Expands the object in from_json to a row whose columns match the record type defined by base (see note below). | select * from json_populate_record(null::myrowtype, '{"a":1,"b":2}') | a | b ---+--- 1 | 2 |
| setof anyelement | Expands the outermost array of objects in from_json to a set of rows whose columns match the record type defined by base (see note below). | select * from json_populate_recordset(null::myrowtype, '[{"a":1,"b":2},{"a":3,"b":4}]') | a | b ---+--- 1 | 2 3 | 4 |
|
| Expands a JSON array to a set of JSON values. | select * from json_array_elements('[1,true, [2,false]]') | value ----------- 1 true [2,false] |
| setof text | Expands a JSON array to a set of text values. | select * from json_array_elements_text('["foo", "bar"]') | value ----------- foo bar |
| text | Returns the type of the outermost JSON value as a text string. Possible types are object, array, string, number, boolean, and null. | json_typeof('-123.4') | number |
| record | Builds an arbitrary record from a JSON object (see note below). As with all functions returning record, the caller must explicitly define the structure of the record with an AS clause. | select * from json_to_record('{"a":1,"b":[1,2,3],"c":"bar"}') as x(a int, b text, d text) | a | b | d ---+---------+--- 1 | [1,2,3] | |
| setof record | Builds an arbitrary set of records from a JSON array of objects (see note below). As with all functions returning record, the caller must explicitly define the structure of the record with an AS clause. | select * from json_to_recordset('[{"a":1,"b":"foo"},{"a":"2","c":"bar"}]') as x(a int, b text); | a | b ---+----- 1 | foo 2 | |
|
| Returns from_json with all object fields that have null values omitted. Other null values are untouched. | json_strip_nulls('[{"f1":1,"f2":null},2,null,3]') | [{"f1":1},2,null,3] |
|
| Returns target with the section designated by path replaced by new_value, or with new_value added if create_missing is true (default is true) and the item designated by path does not exist. As with the path oriented operators, negative integers that appear in path count from the end of JSON arrays. |
|
|
|
| Returns from_json as indented JSON text. | jsonb_pretty('[{"f1":1,"f2":null},2,null,3]') |
[
{
"f1": 1,
"f2": null
},
2,
null,
3
]
|
Note
Many of these functions and operators will convert Unicode escapes in JSON strings to the appropriate single character. This is a non-issue if the input is type jsonb, because the conversion was already done; but for json input, this may result in throwing an error, as noted in Section 8.14.
Note
While the examples for the functions json_populate_record, json_populate_recordset, json_to_record and json_to_recordset use constants, the typical use would be to reference a table in the FROM clause and use one of its json or jsonb columns as an argument to the function. Extracted key values can then be referenced in other parts of the query, like WHERE clauses and target lists. Extracting multiple values in this way can improve performance over extracting them separately with per-key operators.
JSON keys are matched to identical column names in the target row type. JSON type coercion for these functions is “best effort” and may not result in desired values for some types. JSON fields that do not appear in the target row type will be omitted from the output, and target columns that do not match any JSON field will simply be NULL.
Note
All the items of the path parameter of jsonb_set must be present in the target, unless create_missing is true, in which case all but the last item must be present. If these conditions are not met the target is returned unchanged.
If the last path item is an object key, it will be created if it is absent and given the new value. If the last path item is an array index, if it is positive the item to set is found by counting from the left, and if negative by counting from the right - -1 designates the rightmost element, and so on. If the item is out of the range -array_length .. array_length -1, and create_missing is true, the new value is added at the beginning of the array if the item is negative, and at the end of the array if it is positive.
Note
The json_typeof function's null return value should not be confused with a SQL NULL. While calling json_typeof('null'::json) will return null, calling json_typeof(NULL::json) will return a SQL NULL.
Note
If the argument to json_strip_nulls contains duplicate field names in any object, the result could be semantically somewhat different, depending on the order in which they occur. This is not an issue for jsonb_strip_nulls since jsonb values never have duplicate object field names.
See also Section 9.20 for the aggregate function json_agg which aggregates record values as JSON, and the aggregate function json_object_agg which aggregates pairs of values into a JSON object, and their jsonb equivalents, jsonb_agg and jsonb_object_agg.