Re: Log a sample of transactions

Hi,

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.

> <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?

> @@ -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.

As far as I can tell xact_is_sampled is not properly reset in case of
errors?

Greetings,

Andres Freund