PostgreSQL Write JSON File
In the process of working through issues to support ETL transformations from JSON with Python. The first step was extracting a the data from a PostgreSQL table into a JSON file. The syntax wasn’t exactly clear and the PostgreSQL Tutorial was a bit misleading but I worked it out. My initial sample deals with writing the data from the item table of my videodb database to a item.json file in the /tmp directory.
The query retrieves the data but casts the creation_date and last_update_date timestamp data types into date data types, as follows:
SELECT row_to_json( (SELECT ROW FROM (SELECT item_id , item_barcode , item_type , item_title , item_subtitle , item_rating , item_release_date , created_by , creation_date::DATE , last_updated_by , last_update_date::DATE) ROW)) AS DATA FROM item; |
The internal SELECT-list creates the name element of name:value pairs in the JSON structure. Naturally, it does exclude the type casting ::date portion of the query from the name elements. The row_to_json() maps the name list to the list of value results from in each row, and returns a result set.
The query returns the following 21 JSON objects:
{"item_id":1001,"item_barcode":"9736-05640-4","item_type":1016,"item_title":"The Hunt for Red October","item_subtitle":"Special Collector's Edition","item_rating":"PG","item_release_date":"1990-03-02","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1002,"item_barcode":"24543-02392","item_type":1016,"item_title":"Star Wars I","item_subtitle":"Phantom Menace","item_rating":"PG","item_release_date":"1999-05-04","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1003,"item_barcode":"24543-5615","item_type":1015,"item_title":"Star Wars II","item_subtitle":"Attack of the Clones","item_rating":"PG","item_release_date":"2002-05-16","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1004,"item_barcode":"24543-05539","item_type":1016,"item_title":"Star Wars II","item_subtitle":"Attack of the Clones","item_rating":"PG","item_release_date":"2002-05-16","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1005,"item_barcode":"24543-20309","item_type":1016,"item_title":"Star Wars III","item_subtitle":"Revenge of the Sith","item_rating":"PG13","item_release_date":"2005-05-19","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1006,"item_barcode":"86936-70380","item_type":1016,"item_title":"The Chronicles of Narnia","item_subtitle":"The Lion, the Witch and the Wardrobe","item_rating":"PG","item_release_date":"2002-05-16","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1007,"item_barcode":"91493-06475","item_type":1019,"item_title":"RoboCop","item_subtitle":"","item_rating":"Mature","item_release_date":"2003-07-24","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1008,"item_barcode":"93155-11810","item_type":1019,"item_title":"Pirates of the Caribbean","item_subtitle":"","item_rating":"Teen","item_release_date":"2003-06-30","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1009,"item_barcode":"12725-00173","item_type":1019,"item_title":"The Chronicles of Narnia","item_subtitle":"The Lion, the Witch and the Wardrobe","item_rating":"Everyone","item_release_date":"2003-06-30","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1010,"item_barcode":"45496-96128","item_type":1017,"item_title":"MarioKart","item_subtitle":"Double Dash","item_rating":"Everyone","item_release_date":"2003-11-17","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1011,"item_barcode":"08888-32214","item_type":1018,"item_title":"Splinter Cell","item_subtitle":"Chaos Theory","item_rating":"Teen","item_release_date":"2003-04-08","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1012,"item_barcode":"14633-14821","item_type":1018,"item_title":"Need for Speed","item_subtitle":"Most Wanted","item_rating":"Everyone","item_release_date":"2004-11-15","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1013,"item_barcode":"10425-29944","item_type":1019,"item_title":"The DaVinci Code","item_subtitle":"","item_rating":"Teen","item_release_date":"2006-05-19","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1014,"item_barcode":"52919-52057","item_type":1019,"item_title":"Cars","item_subtitle":"","item_rating":"Everyone","item_release_date":"2006-04-28","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1015,"item_barcode":"9689-80547-3","item_type":1020,"item_title":"Beau Geste","item_subtitle":"","item_rating":"PG","item_release_date":"1992-03-01","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1016,"item_barcode":"53939-64103","item_type":1020,"item_title":"I Remember Mama","item_subtitle":"","item_rating":"NR","item_release_date":"1998-01-05","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1017,"item_barcode":"24543-01292","item_type":1020,"item_title":"Tora! Tora! Tora!","item_subtitle":"The Attack on Pearl Harbor","item_rating":"G","item_release_date":"1999-11-02","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1018,"item_barcode":"43396-60047","item_type":1020,"item_title":"A Man for All Seasons","item_subtitle":"","item_rating":"G","item_release_date":"1994-06-28","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1019,"item_barcode":"43396-70603","item_type":1020,"item_title":"Hook","item_subtitle":"","item_rating":"PG","item_release_date":"1991-12-11","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1020,"item_barcode":"85391-13213","item_type":1020,"item_title":"Around the World in 80 Days","item_subtitle":"","item_rating":"G","item_release_date":"1992-12-04","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1021,"item_barcode":"85391-10843","item_type":1020,"item_title":"Camelot","item_subtitle":"","item_rating":"G","item_release_date":"1998-05-15","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"} |
Moving on to the next step you enclose the query in the copy command syntax, like:
COPY (SELECT row_to_json( (SELECT ROW FROM (SELECT item_id , item_barcode , item_type , item_title , item_subtitle , item_rating , item_release_date , created_by , creation_date::DATE , last_updated_by , last_update_date::DATE) ROW)) AS DATA FROM item) TO '/tmp/sample.json'; |
When you run it, it will fail with the following error message:
ERROR: must be superuser or a member of the pg_write_server_files role to COPY to a file HINT: Anyone can COPY to stdout or from stdin. psql's \copy command also works for anyone. |
You can fix this problem by connecting as the postgres user and granting the privilege to the student user:
GRANT pg_write_server_files TO student; |
It then runs successfully as the student user and creates the sample.json file in the /tmp directory. As always, I hope this helps those looking for a solution.
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