52.2. Reporting Errors Within the Server
Error, warning, and log messages generated within the server code should be created using
ereport, or its older cousin
elog. The use of this function is complex enough to require some explanation.
There are two required elements for every message: a severity level (ranging from
PANIC) and a primary message text. In addition there are optional elements, the most common of which is an error identifier code that follows the SQL spec's SQLSTATE conventions.
ereport itself is just a shell function, that exists mainly for the syntactic convenience of making message generation look like a function call in the C source code. The only parameter accepted directly by
ereport is the severity level. The primary message text and any optional message elements are generated by calling auxiliary functions, such as
errmsg, within the
A typical call to
ereport might look like this:
ereport(ERROR, (errcode(ERRCODE_DIVISION_BY_ZERO), errmsg("division by zero")));
This specifies error severity level
ERROR (a run-of-the-mill error). The
errcode call specifies the SQLSTATE error code using a macro defined in
errmsg call provides the primary message text. Notice the extra set of parentheses surrounding the auxiliary function calls — these are annoying but syntactically necessary.
Here is a more complex example:
ereport(ERROR, (errcode(ERRCODE_AMBIGUOUS_FUNCTION), errmsg("function %s is not unique", func_signature_string(funcname, nargs, NIL, actual_arg_types)), errhint("Unable to choose a best candidate function. " "You might need to add explicit typecasts.")));
This illustrates the use of format codes to embed run-time values into a message text. Also, an optional “hint” message is provided.
If the severity level is
ERROR or higher,
ereport aborts the execution of the user-defined function and does not return to the caller. If the severity level is lower than
ereport returns normally.
The available auxiliary routines for
errcode(sqlerrcode)specifies the SQLSTATE error identifier code for the condition. If this routine is not called, the error identifier defaults to
ERRCODE_INTERNAL_ERRORwhen the error severity level is
ERRCODE_WARNINGwhen the error level is
WARNING, otherwise (for
ERRCODE_SUCCESSFUL_COMPLETION. While these defaults are often convenient, always think whether they are appropriate before omitting the
errmsg(const char *msg, ...)specifies the primary error message text, and possibly run-time values to insert into it. Insertions are specified by
sprintf-style format codes. In addition to the standard format codes accepted by
sprintf, the format code
%mcan be used to insert the error message returned by
strerrorfor the current value of
%mdoes not require any corresponding entry in the parameter list for
errmsg. Note that the message string will be run through
gettextfor possible localization before format codes are processed.
errmsg_internal(const char *msg, ...)is the same as
errmsg, except that the message string will not be translated nor included in the internationalization message dictionary. This should be used for “cannot happen” cases that are probably not worth expending translation effort on.
errmsg_plural(const char *fmt_singular, const char *fmt_plural, unsigned long n, ...)is like
errmsg, but with support for various plural forms of the message.
fmt_singularis the English singular format,
fmt_pluralis the English plural format,
nis the integer value that determines which plural form is needed, and the remaining arguments are formatted according to the selected format string. For more information see Section 53.2.2.
errdetail(const char *msg, ...)supplies an optional “detail” message; this is to be used when there is additional information that seems inappropriate to put in the primary message. The message string is processed in just the same way as for
errdetail_internal(const char *msg, ...)is the same as
errdetail, except that the message string will not be translated nor included in the internationalization message dictionary. This should be used for detail messages that are not worth expending translation effort on, for instance because they are too technical to be useful to most users.
errdetail_plural(const char *fmt_singular, const char *fmt_plural, unsigned long n, ...)is like
errdetail, but with support for various plural forms of the message. For more information see Section 53.2.2.
errdetail_log(const char *msg, ...)is the same as
errdetailexcept that this string goes only to the server log, never to the client. If both
errdetail(or one of its equivalents above) and
errdetail_logare used then one string goes to the client and the other to the log. This is useful for error details that are too security-sensitive or too bulky to include in the report sent to the client.
errdetail_log_plural(const char *fmt_singular, const char *fmt_plural, unsigned long n, ...)is like
errdetail_log, but with support for various plural forms of the message. For more information see Section 53.2.2.
errhint(const char *msg, ...)supplies an optional “hint” message; this is to be used when offering suggestions about how to fix the problem, as opposed to factual details about what went wrong. The message string is processed in just the same way as for
errcontext(const char *msg, ...)is not normally called directly from an
ereportmessage site; rather it is used in
error_context_stackcallback functions to provide information about the context in which an error occurred, such as the current location in a PL function. The message string is processed in just the same way as for
errmsg. Unlike the other auxiliary functions, this can be called more than once per
ereportcall; the successive strings thus supplied are concatenated with separating newlines.
errposition(int cursorpos)specifies the textual location of an error within a query string. Currently it is only useful for errors detected in the lexical and syntactic analysis phases of query processing.
errtable(Relation rel)specifies a relation whose name and schema name should be included as auxiliary fields in the error report.
errtablecol(Relation rel, int attnum)specifies a column whose name, table name, and schema name should be included as auxiliary fields in the error report.
errtableconstraint(Relation rel, const char *conname)specifies a table constraint whose name, table name, and schema name should be included as auxiliary fields in the error report. Indexes should be considered to be constraints for this purpose, whether or not they have an associated
pg_constraintentry. Be careful to pass the underlying heap relation, not the index itself, as
errdatatype(Oid datatypeOid)specifies a data type whose name and schema name should be included as auxiliary fields in the error report.
errdomainconstraint(Oid datatypeOid, const char *conname)specifies a domain constraint whose name, domain name, and schema name should be included as auxiliary fields in the error report.
errcode_for_file_access()is a convenience function that selects an appropriate SQLSTATE error identifier for a failure in a file-access-related system call. It uses the saved
errnoto determine which error code to generate. Usually this should be used in combination with
%min the primary error message text.
errcode_for_socket_access()is a convenience function that selects an appropriate SQLSTATE error identifier for a failure in a socket-related system call.
errhidestmt(bool hide_stmt)can be called to specify suppression of the
STATEMENT:portion of a message in the postmaster log. Generally this is appropriate if the message text includes the current statement already.
errhidecontext(bool hide_ctx)can be called to specify suppression of the
CONTEXT:portion of a message in the postmaster log. This should only be used for verbose debugging messages where the repeated inclusion of context would bloat the log volume too much.
At most one of the functions
errdomainconstraint should be used in an
ereport call. These functions exist to allow applications to extract the name of a database object associated with the error condition without having to examine the potentially-localized error message text. These functions should be used in error reports for which it's likely that applications would wish to have automatic error handling. As of PostgreSQL 9.3, complete coverage exists only for errors in SQLSTATE class 23 (integrity constraint violation), but this is likely to be expanded in future.
There is an older function
elog that is still heavily used. An
elog(level, "format string", ...);
is exactly equivalent to:
ereport(level, (errmsg_internal("format string", ...)));
Notice that the SQLSTATE error code is always defaulted, and the message string is not subject to translation. Therefore,
elog should be used only for internal errors and low-level debug logging. Any message that is likely to be of interest to ordinary users should go through
ereport. Nonetheless, there are enough internal “cannot happen” error checks in the system that
elog is still widely used; it is preferred for those messages for its notational simplicity.
Advice about writing good error messages can be found in Section 52.3.
 That is, the value that was current when the
ereport call was reached; changes of
errno within the auxiliary reporting routines will not affect it. That would not be true if you were to write
strerror(errno) explicitly in
errmsg's parameter list; accordingly, do not do so.