Save a few cycles while creating "sticky" entries in pg_stat_statements.

There's no need to sit there and increment the stats when we know all the
increments would be zero anyway.  The actual additions might not be very
expensive, but skipping acquisition of the spinlock seems like a good
thing.  Pushing the logic about initialization of the usage count down into
entry_alloc() allows us to do that while making the code actually simpler,
not more complex.  Expansion on a suggestion by Peter Geoghegan.

contrib/pg_stat_statements/pg_stat_statements.c

@@ -250,7 +250,8 @@ static void pgss_store(const char *query, uint32 queryId,
 		   const BufferUsage *bufusage,
 		   pgssJumbleState * jstate);
 static Size pgss_memsize(void);
-static pgssEntry *entry_alloc(pgssHashKey *key, const char *query, int query_len);
+static pgssEntry *entry_alloc(pgssHashKey *key, const char *query,
+							  int query_len, bool sticky);
 static void entry_dealloc(void);
 static void entry_reset(void);
 static void AppendJumble(pgssJumbleState * jstate,
@@ -502,7 +503,7 @@ pgss_shmem_startup(void)
 												   query_size - 1);
 
 		/* make the hashtable entry (discards old entries if too many) */
-		entry = entry_alloc(&temp.key, buffer, temp.query_len);
+		entry = entry_alloc(&temp.key, buffer, temp.query_len, false);
 
 		/* copy in the actual stats */
 		entry->counters = temp.counters;
@@ -596,7 +597,6 @@ static void
 pgss_post_parse_analyze(ParseState *pstate, Query *query)
 {
 	pgssJumbleState jstate;
-	BufferUsage bufusage;
 
 	/* Assert we didn't do this already */
 	Assert(query->queryId == 0);
@@ -646,16 +646,12 @@ pgss_post_parse_analyze(ParseState *pstate, Query *query)
 	 * there's no need for an early entry.
 	 */
 	if (jstate.clocations_count > 0)
-	{
-		memset(&bufusage, 0, sizeof(bufusage));
-
 		pgss_store(pstate->p_sourcetext,
 				   query->queryId,
 				   0,
 				   0,
-				   &bufusage,
+				   NULL,
 				   &jstate);
-	}
 }
 
 /*
@@ -924,7 +920,7 @@ pgss_hash_string(const char *str)
  *
  * If jstate is not NULL then we're trying to create an entry for which
  * we have no statistics as yet; we just want to record the normalized
- * query string while we can.
+ * query string.  total_time, rows, bufusage are ignored in this case.
  */
 static void
 pgss_store(const char *query, uint32 queryId,
@@ -933,7 +929,6 @@ pgss_store(const char *query, uint32 queryId,
 		   pgssJumbleState * jstate)
 {
 	pgssHashKey key;
-	double		usage;
 	pgssEntry  *entry;
 	char	   *norm_query = NULL;
 
@@ -954,29 +949,7 @@ pgss_store(const char *query, uint32 queryId,
 
 	entry = (pgssEntry *) hash_search(pgss_hash, &key, HASH_FIND, NULL);
 
-	if (jstate)
+	/* Create new entry, if not present */
-	{
-		/*
-		 * When creating an entry just to store the normalized string, make it
-		 * artificially sticky so that it will probably still be there when
-		 * the query gets executed.  We do this by giving it a median usage
-		 * value rather than the normal value.  (Strictly speaking, query
-		 * strings are normalized on a best effort basis, though it would be
-		 * difficult to demonstrate this even under artificial conditions.)
-		 * But if we found the entry already present, don't let this call
-		 * increment its usage.
-		 */
-		if (!entry)
-			usage = pgss->cur_median_usage;
-		else
-			usage = 0;
-	}
-	else
-	{
-		/* normal case, increment usage by normal amount */
-		usage = USAGE_EXEC(duration);
-	}
-
 	if (!entry)
 	{
 		int			query_len;
@@ -999,7 +972,7 @@ pgss_store(const char *query, uint32 queryId,
 			/* Acquire exclusive lock as required by entry_alloc() */
 			LWLockAcquire(pgss->lock, LW_EXCLUSIVE);
 
-			entry = entry_alloc(&key, norm_query, query_len);
+			entry = entry_alloc(&key, norm_query, query_len, true);
 		}
 		else
 		{
@@ -1016,31 +989,26 @@ pgss_store(const char *query, uint32 queryId,
 			/* Acquire exclusive lock as required by entry_alloc() */
 			LWLockAcquire(pgss->lock, LW_EXCLUSIVE);
 
-			entry = entry_alloc(&key, query, query_len);
+			entry = entry_alloc(&key, query, query_len, false);
 		}
 	}
 
-	/*
+	/* Increment the counts, except when jstate is not NULL */
-	 * Grab the spinlock while updating the counters (see comment about
+	if (!jstate)
-	 * locking rules at the head of the file)
-	 */
 	{
+		/*
+		 * Grab the spinlock while updating the counters (see comment about
+		 * locking rules at the head of the file)
+		 */
 		volatile pgssEntry *e = (volatile pgssEntry *) entry;
 
 		SpinLockAcquire(&e->mutex);
 
-		/*
+		/* "Unstick" entry if it was previously sticky */
-		 * If we're entering real data, "unstick" entry if it was previously
+		if (e->counters.calls == 0)
-		 * sticky, and then increment calls.
+			e->counters.usage = USAGE_INIT;
-		 */
-		if (!jstate)
-		{
-			if (e->counters.calls == 0)
-				e->counters.usage = USAGE_INIT;
-
-			e->counters.calls += 1;
-		}
 
+		e->counters.calls += 1;
 		e->counters.total_time += total_time;
 		e->counters.rows += rows;
 		e->counters.shared_blks_hit += bufusage->shared_blks_hit;
@@ -1055,7 +1023,7 @@ pgss_store(const char *query, uint32 queryId,
 		e->counters.temp_blks_written += bufusage->temp_blks_written;
 		e->counters.time_read += INSTR_TIME_GET_DOUBLE(bufusage->time_read);
 		e->counters.time_write += INSTR_TIME_GET_DOUBLE(bufusage->time_write);
-		e->counters.usage += usage;
+		e->counters.usage += USAGE_EXEC(duration);
 
 		SpinLockRelease(&e->mutex);
 	}
@@ -1235,13 +1203,19 @@ pgss_memsize(void)
  *
  * "query" need not be null-terminated; we rely on query_len instead
  *
+ * If "sticky" is true, make the new entry artificially sticky so that it will
+ * probably still be there when the query finishes execution.  We do this by
+ * giving it a median usage value rather than the normal value.  (Strictly
+ * speaking, query strings are normalized on a best effort basis, though it
+ * would be difficult to demonstrate this even under artificial conditions.)
+ *
  * Note: despite needing exclusive lock, it's not an error for the target
  * entry to already exist.	This is because pgss_store releases and
  * reacquires lock after failing to find a match; so someone else could
  * have made the entry while we waited to get exclusive lock.
  */
 static pgssEntry *
-entry_alloc(pgssHashKey *key, const char *query, int query_len)
+entry_alloc(pgssHashKey *key, const char *query, int query_len, bool sticky)
 {
 	pgssEntry  *entry;
 	bool		found;
@@ -1259,7 +1233,8 @@ entry_alloc(pgssHashKey *key, const char *query, int query_len)
 
 		/* reset the statistics */
 		memset(&entry->counters, 0, sizeof(Counters));
-		entry->counters.usage = USAGE_INIT;
+		/* set the appropriate initial usage count */
+		entry->counters.usage = sticky ? pgss->cur_median_usage : USAGE_INIT;
 		/* re-initialize the mutex each time ... we assume no one using it */
 		SpinLockInit(&entry->mutex);
 		/* ... and don't forget the query text */