PG-1674: Fix performance issues with comment extraction

The previous logic used complex regex parsers, which caused performance
issues with large (megabyte sized) queries. This change removes the
regex dependency and uses the same (fixed) logic from the query hashing
code, which is much faster.

It also checks the related GUC variable, which the previous code
ignored: if we do not want to display extracted comments, we won't
extract them in the first place.

This commit doesn't try to address other issues with comment parsing logic:

* we shouldn't treat comment like things within strings as comments
* we should handle nested C style comments
* we don't extract `--` style comments

All of these issues are still there, as before.

pg_stat_monitor.c

@@ -19,7 +19,6 @@
 #include "access/parallel.h"
 #include "nodes/pg_list.h"
 #include "utils/guc.h"
-#include <regex.h>
 #include "pgstat.h"
 #include "commands/dbcommands.h"
 #include "commands/explain.h"
@@ -96,8 +95,6 @@ uint64	   *nested_queryids;
 char	  **nested_query_txts;
 List	   *lentries = NIL;
 
-/* Regex object used to extract query comments. */
-static regex_t preg_query_comments;
 static char relations[REL_LST][REL_LEN];
 
 static int	num_relations;		/* Number of relation in the query */
@@ -280,8 +277,6 @@ static void pgsm_lock_release(pgsmSharedState *pgsm);
 void
 _PG_init(void)
 {
-	int			rc;
-
 	elog(DEBUG2, "[pg_stat_monitor] pg_stat_monitor: %s().", __FUNCTION__);
 
 	/*
@@ -309,18 +304,8 @@ _PG_init(void)
 	EnableQueryId();
 #endif
 
-
 	EmitWarningsOnPlaceholders("pg_stat_monitor");
 
-	/*
-	 * Compile regular expression for extracting out query comments only once.
-	 */
-	rc = regcomp(&preg_query_comments, "/\\*([^*]|[\r\n]|(\\*+([^*/]|[\r\n])))*\\*+/", REG_EXTENDED);
-	if (rc != 0)
-	{
-		elog(ERROR, "[pg_stat_monitor] _PG_init: query comments regcomp() failed, return code=(%d).", rc);
-	}
-
 	/*
 	 * Install hooks.
 	 */
@@ -3977,46 +3962,70 @@ get_histogram_timings(PG_FUNCTION_ARGS)
 	return CStringGetTextDatum(text_str);
 }
 
+
+static bool
+append_comment_char(char *comments, size_t max_len, size_t *idx, char c)
+{
+	if (*idx >= max_len)
+		return false;
+
+	comments[*idx] = c;
+	(*idx)++;
+
+	return true;
+}
+
 static void
 extract_query_comments(const char *query, char *comments, size_t max_len)
 {
-	int			rc;
+	size_t		curr_len = 0;
-	size_t		nmatch = 1;
+	const char *q_iter = query;
-	regmatch_t	pmatch;
-	regoff_t	comment_len,
-				total_len = 0;
-	const char *s = query;
 
-	while (total_len < max_len)
+	if (!pgsm_extract_comments)
 	{
-		rc = regexec(&preg_query_comments, s, nmatch, &pmatch, 0);
+		comments[0] = 0;
-		if (rc != 0)
+		return;
-			break;
+	}
 
-		comment_len = pmatch.rm_eo - pmatch.rm_so;
+	while (q_iter && *q_iter)
-
+	{
-		if (total_len + comment_len > max_len)
+		/*
-			break;				/* TODO: log error in error view, insufficient
+		 * multiline comments, + 1 is safe even if we've reach end of string
-								 * space for comment. */
+		 */
-
+		if (*q_iter == '/' && *(q_iter + 1) == '*')
-		total_len += comment_len;
-
-		/* Not 1st iteration, append ", " before next comment. */
-		if (s != query)
 		{
-			if (total_len + 2 > max_len)
+			if (curr_len != 0)
-				break;			/* TODO: log error in error view, insufficient
+			{
-								 * space for ", " + comment. */
+				if (!append_comment_char(comments, max_len, &curr_len, ','))
+					return;
+				if (!append_comment_char(comments, max_len, &curr_len, ' '))
+					return;
+			}
+			while (*q_iter && *(q_iter + 1) && (*q_iter != '*' || *(q_iter + 1) != '/'))
+			{
+				if (!append_comment_char(comments, max_len, &curr_len, *q_iter))
+					return;
+				q_iter++;
+			}
 
-			memcpy(comments, ", ", 2);
+			if (*q_iter)
-			comments += 2;
+			{
-			total_len += 2;
+				if (!append_comment_char(comments, max_len, &curr_len, *q_iter))
+					return;
+				q_iter++;
+			}
+			if (*q_iter)
+			{
+				if (!append_comment_char(comments, max_len, &curr_len, *q_iter))
+					return;
+				q_iter++;
+			}
 		}
 
-		memcpy(comments, s + pmatch.rm_so, comment_len);
+		q_iter++;
-		comments += comment_len;
-		s += pmatch.rm_eo;
 	}
+
+	comments[curr_len] = 0;
 }
 
 #if PG_VERSION_NUM < 140000