Обсуждение: BUG #17694: In JSONPath expressions, characters between leading $ and dot appear to be ignored

The following bug has been logged on the website:

Bug reference:      17694
Logged by:          David Wheeler
Email address:      david@justatheory.com
PostgreSQL version: 15.1
Operating system:   macOS
Description:

The correct way to specify an absolute JSON path expression is to start with
`$.`, as in:

```
david=# select '{"foo": 1}' @? '$.foo';
 ?column? 
----------
 t
```

If, however, you omit the dot (`.`), the expression incorrectly always
evaluates to true!

```
david=# select '{"foo": 1}' @? '$foo';
 ?column? 
----------
 t

david=# select '{"foo": 1}' @? '$"foo bar"';
 ?column? 
----------
 t

david=# select '{"foo": 1}' @? '$"foo bar".bar';
 ?column? 
----------
 f
```

It looks like the text between the `$` and `.` is ignored. I don't think
this is right. Shouldn't it be a syntax error? Seems to properly complain if
using the same pattern in subpaths:

```
david=# select '{"foo": 1}' @? '$.foo"foo bar"';
ERROR:  syntax error at or near """ of jsonpath input
LINE 1: select '{"foo": 1}' @? '$.foo"foo bar"';
```

On Nov 24, 2022, at 12:16, David G. Johnston <david.g.johnston@gmail.com> wrote:

> There is a bug in this area though the syntax itself is valid since you've simply defined a variable.

Sorry, I don’t follow. What variable is defined? I mean `$` is a variable for the whole expression, and always required
atthe start of a JSONPath, AFAICT. But when running 

    select '{"foo": 1}' @? ‘$foo'

or

    select '{}' @? ‘$foo'

(both of which incorrectly return true), there is no `$foo` path; it has to be `$.foo`.

Best,

David

On Nov 24, 2022, at 12:44, David G. Johnston <david.g.johnston@gmail.com> wrote:

> JSONPath can do/represent much more than you are thinking here.
>
> https://www.postgresql.org/docs/current/datatype-json.html#DATATYPE-JSONPATH
>
> Note the second entry ($varname) in table 8.24

Ah, I see. SQL/JSON syntax gets a bit confusing relative to other flavors.

D

On Nov 24, 2022, at 12:51, David E. Wheeler <david@justatheory.com> wrote:

> Ah, I see. SQL/JSON syntax gets a bit confusing relative to other flavors.

Yeah, this doesn’t seem right, either:

david=# select jsonb_path_exists('{"foo": {"bar": true}}', '$path', '{"path": "$.foo.bax"}');
 jsonb_path_exists
-------------------
 t

D

On Nov 24, 2022, at 12:55, David E. Wheeler <david@justatheory.com> wrote:

> Yeah, this doesn’t seem right, either:
>
> david=# select jsonb_path_exists('{"foo": {"bar": true}}', '$path', '{"path": "$.foo.bax"}');
> jsonb_path_exists
> —————————
> t


david=# select '{}' @? '$.emails.work == "hi@home.com"';
 ?column?
----------
 t

Works properly with `@@`, though:

david=# select '{}' @@ '$.emails.work == "hi@home.com"';
 ?column?
----------
 f

Have to admit I find this pretty weird. I see that the docs say that this Postgres-specific boolean predicate syntax is
requiredfor @@, but it doesn’t say it doesn’t apply to @? But the `?()` filter syntax works as expected with @?: 

david=# select '{}' @? '$.emails.work ?(@ == "hi@home.com")';
 ?column?
----------
 f

Wondering if I should avoid `@?`. Frankly I don’t understand the difference between `@@` and `@?`.

Best,

David