On 2/14/19 9:14 PM, Andres Freund wrote:
Hi,
Hello Andres,
On 2019-01-26 11:44:58 +0100, Adrien NAYRAT wrote:
+ <varlistentry id="guc-transaction-sample-rate"
xreflabel="log_transaction_sample_rate">
+ <term><varname>log_transaction_sample_rate</varname> (<type>real</type>)
+ <indexterm>
+ <primary><varname>log_transaction_sample_rate</varname> configuration
parameter</primary>
+ </indexterm>
+ </term>
+ <listitem>
+ <para>
+ Set the fraction of transactions whose statements are logged. It
applies
+ to each new transaction regardless of their duration. The default is
+ <literal>0</literal>, meaning do not log statements from this
transaction.
+ Setting this to <literal>1</literal> logs all statements for all
transactions.
+ Logging can be disabled inside a transaction by setting this to 0.
+ <varname>log_transaction_sample_rate</varname> is helpful to track a
+ sample of transaction.
+ </para>
+ </listitem>
+ </varlistentry>
+
</variablelist>
I wonder if this doesn't need a warning, explaining that using this when
there are large transactions could lead to slowdowns.
Yes, I will add some wording
<para>
diff --git a/src/backend/access/transam/xact.c
b/src/backend/access/transam/xact.c
index 7c3a9c1e89..1a52c10c39 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -1821,6 +1821,9 @@ StartTransaction(void)
s->state = TRANS_START;
s->transactionId = InvalidTransactionId; /* until assigned */
+ /* Determine if we log statements in this transaction */
+ xact_is_sampled = random() <= log_xact_sample_rate * MAX_RANDOM_VALUE;
+
/*
* initialize current transaction state fields
*
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index e773f20d9f..a11667834e 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -100,7 +100,8 @@ int max_stack_depth = 100;
/* wait N seconds to allow attach from a debugger */
int PostAuthDelay = 0;
-
+/* flag for logging statements in a transaction */
+bool xact_is_sampled = false;
It seems pretty weird to have this in postgres.c, given you declared it
in xact.h?
Yes, I have to revise my C. I will move it to
src/backend/access/transam/xact.c
@@ -2218,7 +2221,8 @@ check_log_statement(List *stmt_list)
int
check_log_duration(char *msec_str, bool was_logged)
{
- if (log_duration || log_min_duration_statement >= 0)
+ if (log_duration || log_min_duration_statement >= 0 ||
+ (xact_is_sampled && log_xact_sample_rate > 0))
Why are both of these checked? ISTM once xact_is_sampled is set, we
ought not to respect a different value of log_xact_sample_rate for the
rest of that transaction.
I added theses checks to allow to disable logging during a sampled
transaction, per suggestion of Masahiko Sawada:
https://www.postgresql.org/message-id/CAD21AoD4t%2BhTV6XfK5Yz%3DEocB8oMyJSYFfjAryCDYtqfib2GrA%40mail.gmail.com
As far as I can tell xact_is_sampled is not properly reset in case of
errors?
Yes, I wonder where we should reset it? Is it in AbortTransaction() or
CleanupTransaction()?
Thanks!
