Are there any way to execute a query inside the string value (like eval) in PostgreSQL?

by

If the statements you are trying to “eval” always return the same data type, you could write an eval() function that uses the EXECUTE mentioned by Grzegorz. 

create or replace function eval(expression text) returns integer
as
$body$
declare
  result integer;
begin
  execute expression into result;
  return result;
end;
$body$
language plpgsql

Then you could do something like 

SELECT eval('select 41') + 1;

But this approach won’t work if your dynamic statements return something different for each expression that you want to evaluate.

Also bear in mind that this opens a huge security risk by running arbitrary statements. If that is a problem depends on your environment. If that is only used in interactive SQL sessions then it isn’t a problem.

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