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 DEBUG to 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 macro that exists mainly for the syntactic convenience of making message generation look like a single 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 ereport call.
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 src/include/utils/errcodes.h. The errmsg call provides the primary message text.
You will also frequently see this older style, with an extra set of parentheses surrounding the auxiliary function calls:
ereport(ERROR,
(errcode(ERRCODE_DIVISION_BY_ZERO),
errmsg("division by zero")));
The extra parentheses were required before PostgreSQL version 12, but are now optional.
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. The auxiliary function calls can be written in any order, but conventionally errcode and errmsg appear first.
If the severity level is ERROR or higher, ereport aborts execution of the current query and does not return to the caller. If the severity level is lower than ERROR, ereport returns normally.
The available auxiliary routines for ereport are:
errcode(sqlerrcode) specifies the SQLSTATE error identifier code for the condition. If this routine is not called, the error identifier defaults to ERRCODE_INTERNAL_ERROR when the error severity level is ERROR or higher, ERRCODE_WARNING when the error level is WARNING, otherwise (for NOTICE and below) ERRCODE_SUCCESSFUL_COMPLETION. While these defaults are often convenient, always think whether they are appropriate before omitting the errcode() call.
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 %m can be used to insert the error message returned by strerror for the current value of errno. [15] %m does not require any corresponding entry in the parameter list for errmsg. Note that the message string will be run through gettext for 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_singular is the English singular format, fmt_plural is the English plural format, n is 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 54.2.2.
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 errmsg.
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 54.2.2.
errdetail_log(const char *msg, ...) is the same as errdetail except that this string goes only to the server log, never to the client. If both errdetail (or one of its equivalents above) and errdetail_log are 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 54.2.2.
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 errmsg.
errcontext(const char *msg, ...) is not normally called directly from an ereport message site; rather it is used in error_context_stack callback 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 ereport call; 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_constraint entry. Be careful to pass the underlying heap relation, not the index itself, as rel.
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 errno to determine which error code to generate. Usually this should be used in combination with %m in 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 too much.
At most one of the functions errtable, errtablecol, errtableconstraint, errdatatype, or 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 call:
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 53.3.
[15] 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.