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Errors and Exceptions

PG2 introduces two kinds of exceptions: PGError and PGErrorResponse.

A PGError exception usually happens on the client side due to wrong values or encode/decode errors. Below, the exception pops up because PG2 cannot encode an Object instance into an integer:

(def stmt
  (pg/prepare conn "select * from users where id = $1"))

(pg/execute-statement conn stmt {:params [(new Object)]})

;; PGError: cannot coerce value to long: type: java.lang.Object...

A PGErrorResponse exception happens when Postgres responds with the ErrorResponse message. Usually it means you have reached a database but it didn't like what you sent. It might be an syntax issue, wrong parameters, or whatever else.

The ErrorResponse message carries plenty of fields describing an error, and the PGErrorResponse class shares them. Namely, the error message renders all the fields received from the server:

(pg/execute conn "selekt 1")

;; PGErrorResponse
;; Server error response: {severity=ERROR, code=42601, file=scan.l,
;;    line=1145,  function=scanner_yyerror, position=1,
;;    message=syntax error at or near \"selekt\", verbosity=ERROR},
;; sql: selekt 1

The message tracks a SQL expression that triggered an error on the server side, if possible (it's unknown sometimes). Above, it's the "sql: selekt 1" part of the message. SQL expressions that exceed 128 characters get truncated with elipsis.

The PGErrorResponse class is compatible with the ex-data function as it implements the clojure.lang.IExceptionInfo interface. Passing an instance of PGErrorResponse into this function returns a Clojure map with all the fields and a full, non-truncated SQL expression:

(try
  (pg/execute conn "selekt 1")
  (catch PGErrorResponse e
    (ex-data e)))

{:verbosity "ERROR"
 :file "scan.l"
 :function "scanner_yyerror"
 :sql "selekt 1"
 :line "1145"
 :severity "ERROR"
 :code "42601"
 :position "1"
 :message "syntax error at or near \"selekt\""}

In rare cases, you can rely on the :code field to manage retries. For example, two transactions let to a conflict, and one of them should be replayed. Here is how you can tackle this case:

(try
  (conn/execute conn "do complex stuff")
  (catch PGErrorResponse e
    (let [code (-> e ex-data :code)]
      (if (= code "123456") ;; a special code from Postgres docs
        (reply-transaction-logic conn)
        (throw e) ;; re-throw it
        ))))