Postgres row_to_json 生成带有双转义引号的无效 JSON

Question

Postgres escapes quotes incorrectly when creating a JSON export. Note the double quotes in the below update...

UPDATE models SET column='"hello"' WHERE id=1;

COPY (SELECT row_to_json(models)
    FROM (SELECT column FROM shaders WHERE id=1) shaders)
    TO '/output.json';

The contents of output.json:

{"column":"\\"hello\\""}

You can see that the quotes are escaped improperly and it creates invalid JSON. It should be:

{"column":"\"hello\""}

How can I fix this Postgres bug or work around it?

Answer 1

This is not JSON related. It's about the way text format (default) in COPY command handles backslashes. From the PostgreSQL documentation - COPY :

Backslash characters ( \\ ) can be used in the COPY data to quote data characters that might otherwise be taken as row or column delimiters. In particular, the following characters must be preceded by a backslash if they appear as part of a column value: backslash itself , newline, carriage return, and the current delimiter character.

(Emphasis mine.)
You can solve it by using CSV-format and changing the quote character from doublequote to something else.

To demonstrate:

SELECT row_to_json(row('"hello"'))
 | "{"f1":"\"hello\""}" |

COPY (SELECT row_to_json(row('"hello"'))) TO '/output.json';
 | {"f1":"\\"hello\\""} |

COPY (SELECT row_to_json(row('"hello"'))) TO '/output.json' CSV QUOTE '$';
 | {"f1":"\"hello\""} |

Answer 2

The answer by Simo Kivistö works if you are certain that the character $ , or whatever the special quote character you chose does not appear in your strings. In my case, I had to export a very large table and there was no particular character which didn't appear in the strings.

To work around this issue, I piped the output of the COPY command to sed to revert the double escaping of quotes:

psql -c "COPY (SELECT row_to_json(t) from my_table as t) to STDOUT;" |
  sed 's/\\"/\"/g' > my_table.json

The sed expression I am piping to simply replaces occurrences of \\\\" with \\" .