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JSON is a popular lightweight and human-readable format for interchanging data. There are so many open-source libraries available for almost all modern programming languages for parsing and consuming JSON data.

I used psql for remotely connecting to the Postgres database and running export statements. To install the Postgres client, run the following in your terminal:. Since our database was hosted on a remote server, we had to first update firewall settings to allow remote connection to the Postgres database server. Run the following command in your terminal to connect to a remote server:.

If the IP and database user name is correct, psql client will prompt you to enter the password for the specified database users. If you do not know the database name, just omit the -d flag and database name from the above command. After connecting to the remote server, run the following commands to list and switch to your intended database:. We want to export the users table into a JSON file for our demo purpose. It can be described as follows:.

There are a lot of columns in users table. To start with the simplest example of dumping all the columns, run the following commands in your terminal:. The first three commands set the tuples mode on, unaligned output format and output file name.

It aggregates the argument provided into a Postgres array. The exported data is saved to the current working directory in a file named users. That's all. Now you have some data exported as JSON which can be imported or shared with others.

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Subscribe to RSS Feed. Need help to launch a new product? I am available for contract work. Hire me to accomplish your business goals with engineering and design.Such data can also be stored as textbut the JSON data types have the advantage of enforcing that each stored value is valid according to the JSON rules.

There are two JSON data types: json and jsonb. They accept almost identical sets of values as input. The major practical difference is one of efficiency.

The json data type stores an exact copy of the input text, which processing functions must reparse on each execution; while jsonb data is stored in a decomposed binary format that makes it slightly slower to input due to added conversion overhead, but significantly faster to process, since no reparsing is needed.

Because the json type stores an exact copy of the input text, it will preserve semantically-insignificant white space between tokens, as well as the order of keys within JSON objects. The processing functions consider the last value as the operative one. By contrast, jsonb does not preserve white space, does not preserve the order of object keys, and does not keep duplicate object keys.

If duplicate keys are specified in the input, only the last value is kept. In general, most applications should prefer to store JSON data as jsonbunless there are quite specialized needs, such as legacy assumptions about ordering of object keys.

PostgreSQL allows only one character set encoding per database. Attempts to directly include characters that cannot be represented in the database encoding will fail; conversely, characters that can be represented in the database encoding but not in UTF8 will be allowed. The fact that the json input function does not make these checks may be considered a historical artifact, although it does allow for simple storage without processing of JSON Unicode escapes in a non-UTF8 database encoding.

Therefore, there are some minor additional constraints on what constitutes valid jsonb data that do not apply to the json type, nor to JSON in the abstract, corresponding to limits on what can be represented by the underlying data type. Notably, jsonb will reject numbers that are outside the range of the PostgreSQL numeric data type, while json will not.

Such implementation-defined restrictions are permitted by RFC However, in practice such problems are far more likely to occur in other implementations, as it is common to represent JSON's number primitive type as IEEE double precision floating point which RFC explicitly anticipates and allows for.

postgres select as json

When using JSON as an interchange format with such systems, the danger of losing numeric precision compared to data originally stored by PostgreSQL should be considered. Conversely, as noted in the table there are some minor restrictions on the input format of JSON primitive types that do not apply to the corresponding PostgreSQL types. The following are all valid json or jsonb expressions:. As previously stated, when a JSON value is input and then printed without any additional processing, json outputs the same text that was input, while jsonb does not preserve semantically-insignificant details such as whitespace.

For example, note the differences here:. One semantically-insignificant detail worth noting is that in jsonbnumbers will be printed according to the behavior of the underlying numeric type. In practice this means that numbers entered with E notation will be printed without it, for example:. However, jsonb will preserve trailing fractional zeroes, as seen in this example, even though those are semantically insignificant for purposes such as equality checks.

Representing data as JSON can be considerably more flexible than the traditional relational data model, which is compelling in environments where requirements are fluid. It is quite possible for both approaches to co-exist and complement each other within the same application. However, even for applications where maximal flexibility is desired, it is still recommended that JSON documents have a somewhat fixed structure.

JSON data is subject to the same concurrency-control considerations as any other data type when stored in a table.Table shows the operators that are available for use with the two JSON data types see Section 8. Note: There are parallel variants of these operators for both the json and jsonb types. The standard comparison operators shown in Table are available for jsonbbut not for json.

They follow the ordering rules for B-tree operations outlined at Section 8.

Some further operators also exist only for jsonbas shown in Table Many of these operators can be indexed by jsonb operator classes. For a full description of jsonb containment and existence semantics, see Section 8.

postgres select as json

Section 8. 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 shows the functions that are available for creating json and jsonb values. Note: The hstore extension has a cast from hstore to jsonso that hstore values converted via the JSON creation functions will be represented as JSON objects, not as primitive string values.

How to Query a JSON Column in PostgreSQL

Table shows the functions that are available for processing json and jsonb values. 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 jsonbbecause the conversion was already done; but for json input, this may result in throwing an error, as noted in Section 8. 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. 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. See also Section 9. If you see anything in the documentation that is not correct, does not match your experience with the particular feature or requires further clarification, please use this form to report a documentation issue.

Development Versions: devel. Unsupported versions: 9. PostgreSQL 9. Functions and Operators Next. Table Throws an error if top level container is not an array. Arrays and composites are converted recursively to arrays and objects; otherwise, if there is a cast from the type to jsonthe 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.

By convention, the argument list consists of alternating keys and values. In all other respects it is identical to the one-argument form. The returned values will be of type text.Today, I experimented around with how to query the data in json columns. Again, jsonb only, we can see if one JSON object contains another. Give me all objects that contain the key "a"with the value 1 associated with that key:.

In this case, we can see that the query object is a superset of the empty object, as well as matching exactly to object 2. We can also filter records that have a matching key-path. In simple cases, using the containment operators might be simpler, but in more complex situations, we would need to use these.

Give me all the records where the value of the element associated with key a is greater than 1. Notice the need to use a text value, rather than a number. Give me objects where element b has a child object that has element c equal to the string "d". There are also versions of these operators that return a text, rather than a json object.

So, all good so far. We can query stuff, and this same stuff can be used to index jsonb columns, too. Key and element existence perform reliably: perhaps surprisingly, the one query will match elements in an array, as well as keys in an object. Note the syntax for a key path: it only allows for strings which json keys must beor integers which array indices are. This seems like a pretty severe limitation.

With django 1. Exact MyModel.Table shows the operators that are available for use with the two JSON data types see Section 8. Note: There are parallel variants of these operators for both the json and jsonb types. The standard comparison operators shown in Table are available for jsonbbut not for json. They follow the ordering rules for B-tree operations outlined at Section 8. Some further operators also exist only for jsonbas shown in Table Many of these operators can be indexed by jsonb operator classes.

For a full description of jsonb containment and existence semantics, see Section 8. Section 8. Table shows the functions that are available for creating json values. Currently, there are no equivalent functions for jsonbbut you can cast the result of one of these functions to jsonb. Note: The hstore extension has a cast from hstore to jsonso that hstore values converted via the JSON creation functions will be represented as JSON objects, not as primitive string values.

Table shows the functions that are available for processing json and jsonb values. 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 jsonbbecause the conversion was already done; but for json input, this may result in throwing an error, as noted in Section 8.

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. See also Section 9. Development Versions: devel. Unsupported versions: 9.

This documentation is for an unsupported version of PostgreSQL. You may want to view the same page for the current version, or one of the supported versions listed above instead. PostgreSQL 9. Functions and Operators Next.

PostgreSQL JSON

Table Arrays and composites are converted recursively to arrays and objects; otherwise, if there is a cast from the type to jsonthe cast function will be used to perform the conversion; otherwise, a JSON scalar value is produced.

For any scalar type other than a number, a Boolean, or a null value, the text representation will be used, properly quoted and escaped so that it is a valid JSON string. By convention, the argument list consists of alternating keys and values. In all other respects it is identical to the one-argument form.

The returned values will be of type text. Possible types are objectarraystringnumberbooleanand null. As with all functions returning recordthe caller must explicitly define the structure of the record with an AS clause. Returns the value as JSON. Returns the array as a JSON array. Returns the row as a JSON object.Such data can also be stored as textbut the JSON data types have the advantage of enforcing that each stored value is valid according to the JSON rules.

Querying JSON (JSONB) data types in PostgreSQL

There are also assorted JSON-specific functions and operators available for data stored in these data types; see Section 9. There are two JSON data types: json and jsonb. They accept almost identical sets of values as input.

The major practical difference is one of efficiency. The json data type stores an exact copy of the input text, which processing functions must reparse on each execution; while jsonb data is stored in a decomposed binary format that makes it slightly slower to input due to added conversion overhead, but significantly faster to process, since no reparsing is needed.

Because the json type stores an exact copy of the input text, it will preserve semantically-insignificant white space between tokens, as well as the order of keys within JSON objects.

The processing functions consider the last value as the operative one. By contrast, jsonb does not preserve white space, does not preserve the order of object keys, and does not keep duplicate object keys.

If duplicate keys are specified in the input, only the last value is kept. In general, most applications should prefer to store JSON data as jsonbunless there are quite specialized needs, such as legacy assumptions about ordering of object keys. PostgreSQL allows only one character set encoding per database.

Attempts to directly include characters that cannot be represented in the database encoding will fail; conversely, characters that can be represented in the database encoding but not in UTF8 will be allowed. The fact that the json input function does not make these checks may be considered a historical artifact, although it does allow for simple storage without processing of JSON Unicode escapes in a non-UTF8 database encoding.

Therefore, there are some minor additional constraints on what constitutes valid jsonb data that do not apply to the json type, nor to JSON in the abstract, corresponding to limits on what can be represented by the underlying data type.

Notably, jsonb will reject numbers that are outside the range of the PostgreSQL numeric data type, while json will not. Such implementation-defined restrictions are permitted by RFC However, in practice such problems are far more likely to occur in other implementations, as it is common to represent JSON's number primitive type as IEEE double precision floating point which RFC explicitly anticipates and allows for.

When using JSON as an interchange format with such systems, the danger of losing numeric precision compared to data originally stored by PostgreSQL should be considered. Conversely, as noted in the table there are some minor restrictions on the input format of JSON primitive types that do not apply to the corresponding PostgreSQL types.Note: The json functions and operators can impose stricter validity requirements than the type's input functions.

In particular, they check much more closely that any use of Unicode surrogate pairs to designate characters outside the Unicode Basic Multilingual Plane is correct. In other encodings the escape sequence must be for an ASCII character, and any other code point in a Unicode escape sequence will result in an error.

postgres select as json

Note: The hstore extension has a cast from hstore to jsonso that converted hstore values are represented as JSON objects, not as string values.

See also Section 9. Development Versions: devel. Unsupported versions: 9. This documentation is for an unsupported version of PostgreSQL. You may want to view the same page for the current version, or one of the supported versions listed above instead. PostgreSQL 9. Functions and Operators Next. Table If the data type is not built in, and there is a cast from the type to jsonthe cast function will be used to perform the conversion.

Otherwise, for any value other than a number, a Boolean, or a null value, the text representation will be used, escaped and quoted so that it is legal JSON. The returned value will be of type text. Only the "outer" object will be displayed. If a column is specified more than once, the last value is used. Returns the array as JSON. Returns the row as JSON. Returns the value as JSON. Returns set of keys in the JSON object.