The SQL-standard's SQL/JSON description uses the term JSON PATH but does not refer to XPath. The postgres docs however say that jsonpath is inspired by XPath.

"SQL/JSON path expressions specify the items to be retrieved from the JSON data, similar to XPath expressions used for SQL access to XML"

Here is an example. The JSON table that I use here consists of millions of rows, each row with a so-called JSON document, not very large, and not very deeply nested. It seems to me that this is/will be typical usage. I retrieved test-JSON data from https://extendsclass.com/json-generator.html (h/t choroba).

One example row with its JSON content: 

{                                       
    "age": 63,                          
    "city": "New York City",            
    "date": "1986-04-03",               
    "array": [                          
        "Lila",                         
        "Brandise",                     
        "Jsandye"                       
    ],                                  
    "country": "Iceland",               
    "friends": [                        
        {                               
            "country": "CA",            
            "lastname": "Yusuk",        
            "firstname": "Deirdre"      
        },                              
        {                               
            "country": "AZ",            
            "lastname": "Suanne",       
            "firstname": "Elfreda"      
        },                              
        {                               
            "country": "FM",            
            "lastname": "Magdalen",     
            "firstname": "Britte"       
        }                               
    ],                                  
    "lastname": "Hailee",               
    "firstname": "Jaime",               
    "countryCode": "IE",                
}

[download]

Here is a query example. The @@ is a postgresql operator, but the part in the single quotes is a jsonpath expression which is SQL/JSON-compliant. 

select id, js->'friends' as friends
from jsontable
where js @@ '$.friends[*].country == "AZ"
  && $.friends[*].lastname starts with "Y" ' ;


   id    |               friends              
---------+-------------------------------------
  110877 | [                                   
         |     {                               
         |         "country": "CA",            
         |         "lastname": "Yusuk",        
         |         "firstname": "Deirdre"      
         |     },                              
         |     {                               
         |         "country": "AZ",            
         |         "lastname": "Suanne",       
         |         "firstname": "Elfreda"      
         |     },                              
         |     {                               
         |         "country": "FM",            
         |         "lastname": "Magdalen",     
         |         "firstname": "Britte"       
         |     }                               
         | ]
  116322 | [                                   
         |     {                               
         |         "country": "AZ",            
         |         "lastname": "Ietta",        
         |         "firstname": "Wendi"        
         |     },                              
         |     {                               
         |         "country": "NG",            
         |         "lastname": "Yuille",       
         |         "firstname": "Giustina"     
         |     },                              
         |     {                               
         |         "country": "MS",            
         |         "lastname": "Tengdin",      
         |         "firstname": "Adelle"       
         |     }                               
         | ]
  117778 | [                                   
         |     {                               
         |         "country": "CK",            
         |         "lastname": "Fancie",       
         |         "firstname": "Fanny"        
         |     },                              
         |     {                               
         |         "country": "JM",            
         |         "lastname": "Jerald",       
         |         "firstname": "Mureil"       
         |     },                              
         |     {                               
         |         "country": "AZ",            
         |         "lastname": "Yam",          
         |         "firstname": "Wileen"       
         |     }                               
         | ]

[[...]]

[download]

This query returns ~1000 rows (above only 3 are shown) from a 3 GB table with 3 M rows. Thanks to an index on the JSON column (js), the query returns in 120 ms.

YMMV. It is probably more interesting to try on a real instance. I did some fiddle-ing:

db-fiddle, postgres 13

https://www.postgresql.org/docs/current/datatype-json.html#DATATYPE-JSONPATH

In reply to Re^4: [OT] Re^3: Examples or tutorials for Perl grammars? by erix
in thread Examples or tutorials for Perl grammars? by LittleJack

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