On 05/02/2026 11:22, Álvaro Herrera wrote:
> My only comment at this point is that the proposed GUC name is not
> great. I think it should be something like log_statement_max_length, or
> something like that. Reading just the thread subject, people would
> imagine this is about the size of the log file.
Done. GUC changed to log_statement_max_length.
> Another point is that the current patch does strlen() twice on each
> query. It might be better to do away with need_truncate_query_log() and
> have a single routine that both determines whether the truncation is
> needed and returns the truncated query if it is. If it returns NULL
> then caller assumes it's not needed.
Done. Now truncate_query_log() returns a palloc'd truncated copy of the
statement if truncation is needed, and NULL otherwise.
== issue with issue with empty STATEMENT ==
This issue was mentioned by Fujii upthread. It is now fixed:
postgres=# SET log_statement TO 'all';
SET
postgres=# SET log_statement_max_length TO 3;
SET
postgres=# SELECT 1/0;
ERROR: division by zero
Log entries:
2026-02-05 17:54:47.521 CET [570568] LOG: statement: SEL
2026-02-05 17:54:47.521 CET [570568] ERROR: division by zero
2026-02-05 17:54:47.521 CET [570568] STATEMENT: SELECT 1/0;
== default value ==
To be consistent with other parameters, such as
log_parameter_max_length, the default (disabled) value is now -1.
== tests ==
Added new tests for -1 and multi-byte characters.
Best, Jim
From d12a565220d1827079c033507919ac5229004ce7 Mon Sep 17 00:00:00 2001
From: Jim Jones <[email protected]>
Date: Thu, 5 Feb 2026 18:00:13 +0100
Subject: [PATCH v7] Add log_statement_max_length GUC to limit logged statement
size
When log_statement is enabled, queries can be arbitrarily long and may
consume significant disk space in server logs. This patch introduces a
new GUC, log_statement_max_length, which limits the maximum byte length
of logged statements.
A value greater than zero truncates each logged statement to the given
number of bytes. The default is -1, which disables truncation and logs
full statements. If specified without units, the value is interpreted
as bytes.
---
doc/src/sgml/config.sgml | 18 ++++++++++
src/backend/tcop/postgres.c | 11 ++++--
src/backend/utils/error/elog.c | 34 +++++++++++++++++++
src/backend/utils/misc/guc_parameters.dat | 10 ++++++
src/backend/utils/misc/guc_tables.c | 1 +
src/backend/utils/misc/postgresql.conf.sample | 2 ++
src/bin/pg_ctl/t/004_logrotate.pl | 24 +++++++++++++
src/include/utils/elog.h | 2 ++
src/include/utils/guc.h | 1 +
9 files changed, 101 insertions(+), 2 deletions(-)
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 5560b95ee6..d9d708d7b9 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8235,6 +8235,24 @@ log_line_prefix = '%m [%p] %q%u@%d/%a '
</listitem>
</varlistentry>
+ <varlistentry id="guc-log-statement-max-length" xreflabel="log_statement_max_length">
+ <term><varname>log_statement_max_length</varname> (<type>integer</type>)
+ <indexterm>
+ <primary><varname>log_statement_max_length</varname> configuration parameter</primary>
+ </indexterm>
+ </term>
+ <listitem>
+ <para>
+ If greater than zero, each statement logged by <xref linkend="guc-log-statement"/>
+ is truncated to at most this many bytes.
+ <literal>-1</literal> (the default) disables truncation.
+ If this value is specified without units, it is taken as bytes.
+ Only superusers and users with the appropriate <literal>SET</literal>
+ privilege can change this setting.
+ </para>
+ </listitem>
+ </varlistentry>
+
</variablelist>
</sect2>
<sect2 id="runtime-config-logging-csvlog">
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 02e9aaa6bc..14265ecd08 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -72,6 +72,7 @@
#include "tcop/pquery.h"
#include "tcop/tcopprot.h"
#include "tcop/utility.h"
+#include "utils/elog.h"
#include "utils/guc_hooks.h"
#include "utils/injection_point.h"
#include "utils/lsyscache.h"
@@ -1023,11 +1024,15 @@ exec_simple_query(const char *query_string)
bool was_logged = false;
bool use_implicit_block;
char msec_str[32];
+ char *truncated_query = NULL;
+ const char *query_log;
/*
* Report query to various monitoring facilities.
*/
debug_query_string = query_string;
+ truncated_query = truncate_query_log(query_string);
+ query_log = truncated_query ? truncated_query : query_string;
pgstat_report_activity(STATE_RUNNING, query_string);
@@ -1072,7 +1077,7 @@ exec_simple_query(const char *query_string)
if (check_log_statement(parsetree_list))
{
ereport(LOG,
- (errmsg("statement: %s", query_string),
+ (errmsg("statement: %s", query_log),
errhidestmt(true),
errdetail_execute(parsetree_list)));
was_logged = true;
@@ -1370,7 +1375,7 @@ exec_simple_query(const char *query_string)
case 2:
ereport(LOG,
(errmsg("duration: %s ms statement: %s",
- msec_str, query_string),
+ msec_str, query_log),
errhidestmt(true),
errdetail_execute(parsetree_list)));
break;
@@ -1381,6 +1386,8 @@ exec_simple_query(const char *query_string)
TRACE_POSTGRESQL_QUERY_DONE(query_string);
+ if (truncated_query)
+ pfree(truncated_query);
debug_query_string = NULL;
}
diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index e6a4ef9905..ffabbfbceb 100644
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -3805,6 +3805,40 @@ write_stderr(const char *fmt,...)
va_end(ap);
}
+/*
+ * truncate_query_log - truncate query string if needed for logging
+ *
+ * Returns a palloc'd truncated copy if truncation is needed,
+ * or NULL if no truncation is required.
+ */
+char *
+truncate_query_log(const char *query)
+{
+ size_t query_len;
+ size_t truncated_len;
+ char *truncated_query;
+
+ /* Check if truncation is disabled (-1) or no query string provided */
+ if (!query || log_statement_max_length < 0)
+ return NULL;
+
+ query_len = strlen(query);
+
+ /*
+ * No need to allocate a truncated copy if the query is shorter
+ * than log_statement_max_length.
+ */
+ if (query_len <= (size_t) log_statement_max_length)
+ return NULL;
+
+ /* Truncate at multibyte character boundary */
+ truncated_len = pg_mbcliplen(query, query_len, log_statement_max_length);
+ truncated_query = (char *) palloc(truncated_len + 1);
+ memcpy(truncated_query, query, truncated_len);
+ truncated_query[truncated_len] = '\0';
+
+ return truncated_query;
+}
/*
* Write errors to stderr (or by equal means when stderr is
diff --git a/src/backend/utils/misc/guc_parameters.dat b/src/backend/utils/misc/guc_parameters.dat
index f0260e6e41..4864dbbbb2 100644
--- a/src/backend/utils/misc/guc_parameters.dat
+++ b/src/backend/utils/misc/guc_parameters.dat
@@ -1777,6 +1777,16 @@
options => 'log_statement_options',
},
+{ name => 'log_statement_max_length', type => 'int', context => 'PGC_SUSET', group => 'LOGGING_WHAT',
+ short_desc => 'Sets the maximum length in bytes of logged statements.',
+ long_desc => '-1 means no truncation.',
+ flags => 'GUC_UNIT_BYTE',
+ variable => 'log_statement_max_length',
+ boot_val => '-1',
+ min => '-1',
+ max => 'INT_MAX / 2',
+},
+
{ name => 'log_statement_sample_rate', type => 'real', context => 'PGC_SUSET', group => 'LOGGING_WHEN',
short_desc => 'Fraction of statements exceeding "log_min_duration_sample" to be logged.',
long_desc => 'Use a value between 0.0 (never log) and 1.0 (always log).',
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 13c569d879..dbcb25e130 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -544,6 +544,7 @@ int log_min_duration_statement = -1;
int log_parameter_max_length = -1;
int log_parameter_max_length_on_error = 0;
int log_temp_files = -1;
+int log_statement_max_length = -1;
double log_statement_sample_rate = 1.0;
double log_xact_sample_rate = 0;
char *backtrace_functions;
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index c4f92fcdac..3cc4399026 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -642,6 +642,8 @@
# bind-parameter values to N bytes;
# -1 means print in full, 0 disables
#log_statement = 'none' # none, ddl, mod, all
+#log_statement_max_length = -1 # max length of logged statements
+ # -1 disables truncation
#log_replication_commands = off
#log_temp_files = -1 # log temporary files equal or larger
# than the specified size in kilobytes;
diff --git a/src/bin/pg_ctl/t/004_logrotate.pl b/src/bin/pg_ctl/t/004_logrotate.pl
index 7b19f86467..dd1aa21f4c 100644
--- a/src/bin/pg_ctl/t/004_logrotate.pl
+++ b/src/bin/pg_ctl/t/004_logrotate.pl
@@ -135,6 +135,30 @@ check_log_pattern('stderr', $new_current_logfiles, 'syntax error', $node);
check_log_pattern('csvlog', $new_current_logfiles, 'syntax error', $node);
check_log_pattern('jsonlog', $new_current_logfiles, 'syntax error', $node);
+# Verify truncation works with ASCII
+$node->append_conf('postgresql.conf', "log_statement = 'all'\nlog_statement_max_length = 20\n");
+$node->reload();
+$node->psql('postgres', "SELECT '123456789ABCDEF'");
+check_log_pattern('stderr', $new_current_logfiles, "SELECT '123456789ABC", $node);
+check_log_pattern('csvlog', $new_current_logfiles, "SELECT '123456789ABC", $node);
+check_log_pattern('jsonlog', $new_current_logfiles, "SELECT '123456789ABC", $node);
+
+# Verify -1 disables truncation (logs full query)
+$node->append_conf('postgresql.conf', "log_statement_max_length = -1\n");
+$node->reload();
+$node->psql('postgres', "SELECT '123456789ABCDEF'");
+check_log_pattern('stderr', $new_current_logfiles, "SELECT '123456789ABCDEF'", $node);
+check_log_pattern('csvlog', $new_current_logfiles, "SELECT '123456789ABCDEF'", $node);
+check_log_pattern('jsonlog', $new_current_logfiles, "SELECT '123456789ABCDEF'", $node);
+
+# Verify multibyte character handling (must not produce invalid UTF-8)
+$node->append_conf('postgresql.conf', "log_statement_max_length = 12\n");
+$node->reload();
+$node->psql('postgres', "SELECT '🐘test'");
+check_log_pattern('stderr', $new_current_logfiles, "SELECT '", $node);
+check_log_pattern('csvlog', $new_current_logfiles, "SELECT '", $node);
+check_log_pattern('jsonlog', $new_current_logfiles, "SELECT '", $node);
+
$node->stop();
done_testing();
diff --git a/src/include/utils/elog.h b/src/include/utils/elog.h
index a12b379e09..aabee73aa4 100644
--- a/src/include/utils/elog.h
+++ b/src/include/utils/elog.h
@@ -494,6 +494,7 @@ extern PGDLLIMPORT int Log_destination;
extern PGDLLIMPORT char *Log_destination_string;
extern PGDLLIMPORT bool syslog_sequence_numbers;
extern PGDLLIMPORT bool syslog_split_messages;
+extern PGDLLIMPORT int log_statement_max_length;
/* Log destination bitmap */
#define LOG_DESTINATION_STDERR 1
@@ -508,6 +509,7 @@ extern void log_status_format(StringInfo buf, const char *format,
extern void DebugFileOpen(void);
extern char *unpack_sql_state(int sql_state);
extern bool in_error_recursion_trouble(void);
+extern char *truncate_query_log(const char *query);
/* Common functions shared across destinations */
extern void reset_formatted_start_time(void);
diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
index bf39878c43..8043d8fa45 100644
--- a/src/include/utils/guc.h
+++ b/src/include/utils/guc.h
@@ -300,6 +300,7 @@ extern PGDLLIMPORT int client_min_messages;
extern PGDLLIMPORT int log_min_duration_sample;
extern PGDLLIMPORT int log_min_duration_statement;
extern PGDLLIMPORT int log_temp_files;
+extern PGDLLIMPORT int log_statement_max_length;
extern PGDLLIMPORT double log_statement_sample_rate;
extern PGDLLIMPORT double log_xact_sample_rate;
extern PGDLLIMPORT char *backtrace_functions;
--
2.43.0
