Allow sampling of statements depending on duration

This allows logging a sample of statements, without incurring excessive
log traffic (which may impact performance).  This can be useful when
analyzing workloads with lots of short queries.

The sampling is configured using two new GUC parameters:

 * log_min_duration_sample - minimum required statement duration

 * log_statement_sample_rate - sample rate (0.0 - 1.0)

Only statements with duration exceeding log_min_duration_sample are
considered for sampling. To enable sampling, both those GUCs have to
be set correctly.

The existing log_min_duration_statement GUC has a higher priority, i.e.
statements with duration exceeding log_min_duration_statement will be
always logged, irrespectedly of how the sampling is configured. This
means only configurations

  log_min_duration_sample < log_min_duration_statement

do actually sample the statements, instead of logging everything.

Author: Adrien Nayrat
Reviewed-by: David Rowley, Vik Fearing, Tomas Vondra
Discussion: https://postgr.es/m/bbe0a1a8-a8f7-3be2-155a-888e661cc06c@anayrat.info

src/backend/utils/misc/postgresql.conf.sample

@@ -493,6 +493,17 @@
 					# statements running at least this number
 					# of milliseconds
 
+#log_min_duration_sample = -1	# -1 is disabled, 0 logs a sample of statements
+					# and their durations, > 0 logs only a sample of
+					# statements running at least this number
+					# of milliseconds
+					# Sample fraction is determined by log_statement_sample_rate
+
+#log_statement_sample_rate = 1.0   # Fraction of logged statements exceeding
+                   # log_min_duration_sample to be logged.
+                   # 1.0 logs all such statements, 0.0 never logs.
+
+
 #log_transaction_sample_rate = 0.0	# Fraction of transactions whose statements
 					# are logged regardless of their duration. 1.0 logs all
 					# statements from all transactions, 0.0 never logs.