How to extract a particular element from a json array in postgres? - arrays

Question

I have a table parameter having 2 columns id(integer) and param_specs(text).
the actual param_specs column looks like above pic (to simplify it check below:-
)
param_specs
[
{"paramName":"param1",
"type":"string",
"defaultValue":"tomcat7",
"optional":false,
"deploymentParam":false},
{"paramName":"param123PreStopAction",
"type":"path",
"defaultValue":"HELLO",
"optional":false,
"deploymentParam":false}
]
So it is an array of json array and i want to fetch the defaultValue field of paramName param123PreStopAction i.e. HELLO.
****EDIT****
As can be seen in the image this is what my table called parameter looks like having two columns I want to get defaultValue of each row in parameter table where paramName LIKE (%PostStopAction) or (%PreStopAction) check the bold values in image(i.e. the paramName should have either PreStopAction or PostStopAction within the actual paramName value eg 'mytomcat7PostStopAction' and fetch its defaultValue i.e 'post-stop'.)
There can be some rows in the table where there wont be any json having preStop or PostStop paramName like row 3 in the image
can someone help me with the query?
As JGH suggested something as follows:-
SELECT "defaultValue"
FROM parameter a
CROSS JOIN LATERAL
json_to_recordset(a.param_spec::json) AS x("paramName" text,"defaultValue" text)
WHERE "paramName”LIKE “%PreStopAction' OR “paramName” LIKE “%PostStopAction”

Answer 1

One approach is to explode your array in fields and to query them. The trick is to consider only the fields of interest.
Select myOutputField
from json_to_recordset('[the array]') as (myQueryField text, myOutputField text)
where myQueryField = myCondition;
Or, bound to your example:
select "defaultValue" from json_to_recordset('
[
{"paramName":"param1",
"type":"string",
"defaultValue":"tomcat7",
"optional":false,
"deploymentParam":false},
{"paramName":"param123PreStopAction",
"type":"path",
"defaultValue":"HELLO",
"optional":false,
"deploymentParam":false}
]') as x("paramName" text,"defaultValue" text)
where "paramName" = 'param123PreStopAction';
** EDIT **
Your data is not saved in a json column but in a text column. You would have to convert it to json (ideally, the column itself... or at least its content). Also, the json_to_recordset works on single items, not on sets, so you would need to use a LATERAL JOIN to overcome this limitation, as nicely explained here.
SELECT myOutputField
FROM mytable a
CROSS JOIN LATERAL
json_to_recordset(a.jsonintextcolumn::json) as (myQueryField text, myOutputField text)
WHERE myQueryField = myCondition;
Or, bound to your example:
SELECT "defaultValue"
FROM public.testjsontxt a
CROSS JOIN LATERAL
json_to_recordset(a.param_specs::json) as x("paramName" text,"defaultValue" text)
WHERE "paramName" = 'param123PreStopAction';