PG -156: replace query placeholders with actual arguments for prepared statements (#481)

* Denormalize prepared statement queries

Added support for extracting query arguments for prepared statements
when `pg_stat_monitor.pgsm_normalized_query` is off.

Previously pg_stat_monitor was unable to extract the arguments for
prepared statements, thus leaving queries with placeholders $1
.. $N instead of the actual arguments.

* Optmize query denormalization

Instead of copying original query text byte by byte, copy data between
query placeholders in chunks, example:

`INSERT INTO foo(a, b, c) VALUES('test', 100, 'test again)'`

Would result in normalized query:

`INSERT INTO foo(a, b, c) VALUES($1, $2, $3)`

The original patch would copy the parts between placeholders byte by
byte, e.g. `INSERT INTO foo(a, b, c) VALUES(`, instead we can copy this
whole block at once, 1 function call and maybe 1 buffer re-allocation
per call.

Also make use of `appendBinaryStringInfo` to avoid calculating string
length as we have this info already.

* Optmize query denormalization(2)

Avoid allocating an array of strings for extracting query argument
values, instead append the current parameter value directly in the
buffer used to store the denormalized query.

This avoids not only unnecessary memory allocations, but also copying
data between temporary memory and the buffer.

* Store denormalized query only under certain constraints

This commit introduces a little optimization along with a feature, it
stores the query in denormalized form only under the circumstances
below:

- The psgm_normalized_query GUC is disabled (off).
- The query is seem for the first time, or the query total
  execution time exceeds the mean execution time calculated for
  the previous queries.

Having the query which took most execution time along with it's
arguments could help users in further investigating performance issues.

* Fix regression tests

When query normalization is disabled utility queries like SELECT 10+20
are now stored as is, instead of SELECT $1+$2.

Also when functions or sub queries are created the arguments used
internally by the function or subqueries will be replaced by NULL instead
of $1..$N. The actual arguments will be displayed when the function or
subquery is actually invoked.

* Add query denormalization regression test for prepared statements

Ensures that the denormalization of prepared statements is working, also
ensure that a query which takes more time to execute replaces the
previous denormalized query.

* Updated pgsm_query_id regression tests

With the query dernomalization feature, having integer literals used in
sql like 1, or 2 could create some confusion on whether those are
placeholders or constant values, thus this commit updates the
pgsm_query_id regression test to use different integer literals to avoid
confusion.

* Improve query denormalization regression test

Add a new test case:

1. Execute a prepared statement with larger execution time first.
2. Execute the same prepared statement with cheap execution time.
3. Ensures that the denormalized heavy query is not replaced by the
   cheaper.

* Format source using pgindent

* Fix top query regression tests on PG 12,13

On PG 12, 13, the internal return instruction in the following function:
```
CREATE OR REPLACE FUNCTION add(int, int) RETURNS INTEGER AS
  $$
  BEGIN
     return (select $1 + $2);
  END; $$ language plpgsql;
```

Is stored as SELECT (select expr1 + expr2)

On PG 14 onward it's stored just as SELECT (expr1 + expr2)

pg_stat_monitor.c

@@ -20,9 +20,11 @@
 #include "nodes/pg_list.h"
 #include "utils/guc.h"
 #include <regex.h>
+#include <stddef.h>
 #include "pgstat.h"
 #include "commands/dbcommands.h"
 #include "commands/explain.h"
+#include "lib/stringinfo.h"
 #include "pg_stat_monitor.h"
 
  /*
@@ -206,6 +208,10 @@ static void pgsm_add_to_list(pgsmEntry *entry, char *query_text, int query_len);
 static pgsmEntry *pgsm_get_entry_for_query(uint64 queryid, PlanInfo *plan_info, const char *query_text, int query_len, bool create);
 static uint64 get_pgsm_query_id_hash(const char *norm_query, int len);
 
+static void get_param_value(const ParamListInfo plist, int idx, StringInfoData *buffer);
+
+static StringInfoData get_denormalized_query(const ParamListInfo paramlist, const char *query_text);
+
 static void pgsm_cleanup_callback(void *arg);
 static void pgsm_store_error(const char *query, ErrorData *edata);
 
@@ -232,7 +238,14 @@ static void pgsm_update_entry(pgsmEntry *entry,
 							  const struct JitInstrumentation *jitusage,
 							  bool reset,
 							  pgsmStoreKind kind);
-static void pgsm_store(pgsmEntry *entry);
+static void pgsm_store_ex(pgsmEntry *entry, ParamListInfo params);
+
+/* Stores query entry in normalized form */
+static inline void
+pgsm_store(pgsmEntry *entry)
+{
+	pgsm_store_ex(entry, NULL);
+}
 
 static void pg_stat_monitor_internal(FunctionCallInfo fcinfo,
 									 pgsmVersion api_version,
@@ -692,12 +705,12 @@ pgsm_ExecutorEnd(QueryDesc *queryDesc)
 	PlanInfo	plan_info;
 	PlanInfo   *plan_ptr = NULL;
 	pgsmEntry  *entry = NULL;
+	MemoryContext oldctx;
 
 	/* Extract the plan information in case of SELECT statement */
 	if (queryDesc->operation == CMD_SELECT && pgsm_enable_query_plan)
 	{
 		int			rv;
-		MemoryContext oldctx;
 
 		/*
 		 * Making sure it is a per query context so that there's no memory
@@ -775,7 +788,7 @@ pgsm_ExecutorEnd(QueryDesc *queryDesc)
 						  false,	/* reset */
 						  PGSM_EXEC);	/* kind */
 
-		pgsm_store(entry);
+		pgsm_store_ex(entry, queryDesc->params);
 	}
 
 	if (prev_ExecutorEnd)
@@ -1861,7 +1874,6 @@ pgsm_create_hash_entry(uint64 bucket_id, uint64 queryid, PlanInfo *plan_info)
 	return entry;
 }
 
-
 /*
  * Store some statistics for a statement.
  *
@@ -1871,9 +1883,15 @@ pgsm_create_hash_entry(uint64 bucket_id, uint64 queryid, PlanInfo *plan_info)
  * 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.  total_time, rows, bufusage are ignored in this case.
+ *
+ * If params argument is not null and pgsm_normalized_query is off then we
+ * denormalize the query using it's actual arguments found in params.
+ * The denormalization is done during the first time the query is
+ * inserted or if the time to execute the query exceeds the average
+ * time computed for the same query.
  */
-static void
-pgsm_store(pgsmEntry *entry)
+void
+pgsm_store_ex(pgsmEntry *entry, ParamListInfo params)
 {
 	pgsmEntry  *shared_hash_entry;
 	pgsmSharedState *pgsm;
@@ -1888,6 +1906,7 @@ pgsm_store(pgsmEntry *entry)
 	JitInstrumentation jitusage;
 	char		comments[COMMENTS_LEN] = {0};
 	int			comments_len;
+	StringInfoData query_info;
 
 	/* Safety check... */
 	if (!IsSystemInitialized())
@@ -1976,6 +1995,14 @@ pgsm_store(pgsmEntry *entry)
 		dsa_area   *query_dsa_area;
 		char	   *query_buff;
 
+		/* Denormalize the query if normalization is off */
+		if (!pgsm_normalized_query && params != NULL)
+		{
+			query_info = get_denormalized_query(params, query);
+			query = query_info.data;
+			query_len = query_info.len;
+		}
+
 		/* New query, truncate length if necessary. */
 		if (query_len > pgsm_query_max_len)
 			query_len = pgsm_query_max_len;
@@ -2065,6 +2092,50 @@ pgsm_store(pgsmEntry *entry)
 		snprintf(shared_hash_entry->username, sizeof(shared_hash_entry->username), "%s", entry->username);
 	}
 
+	/*
+	 * Entry already exists, if query normalization is disabled and the query
+	 * execution time exceeds the mean time for this query, then we
+	 * denormalize the query so users can inspect which arguments caused the
+	 * query to take more time to execute
+	 */
+	else if (
+			 !pgsm_normalized_query &&
+			 params != NULL &&
+			 entry->counters.time.total_time > shared_hash_entry->counters.time.mean_time
+		)
+	{
+		dsa_pointer dsa_query_pointer;
+		dsa_area   *query_dsa_area;
+		char	   *query_buff;
+
+		query_info = get_denormalized_query(params, query);
+		query = query_info.data;
+		query_len = query_info.len;
+
+		/* truncate length if necessary. */
+		if (query_len > pgsm_query_max_len)
+			query_len = pgsm_query_max_len;
+
+		/* Save the query text in raw dsa area */
+		query_dsa_area = get_dsa_area_for_query_text();
+		dsa_query_pointer = dsa_allocate_extended(query_dsa_area, query_len + 1, DSA_ALLOC_NO_OOM | DSA_ALLOC_ZERO);
+		if (DsaPointerIsValid(dsa_query_pointer))
+		{
+			/*
+			 * Get the memory address from DSA pointer and copy the query text
+			 * to it.
+			 */
+			query_buff = dsa_get_address(query_dsa_area, dsa_query_pointer);
+			memcpy(query_buff, query, query_len);
+
+			/* release previous query from shared memory */
+			if (DsaPointerIsValid(shared_hash_entry->query_text.query_pos))
+				dsa_free(query_dsa_area, shared_hash_entry->query_text.query_pos);
+
+			shared_hash_entry->query_text.query_pos = dsa_query_pointer;
+		}
+	}
+
 	pgsm_update_entry(shared_hash_entry,	/* entry */
 					  query,	/* query */
 					  comments, /* comments */
@@ -4018,3 +4089,89 @@ get_query_id(JumbleState *jstate, Query *query)
 	return queryid;
 }
 #endif
+
+/*
+ * extract parameter value (Datum) from plist->params[idx], cast it to string then
+ * append the resulting string to the buffer.
+ */
+void
+get_param_value(const ParamListInfo plist, int idx, StringInfoData *buffer)
+{
+	Oid			typoutput;
+	bool		typisvarlena;
+	char	   *pstring;
+	ParamExternData *param;
+
+	Assert(idx < plist->numParams);
+
+	param = &plist->params[idx];
+
+	if (param->isnull || !OidIsValid(param->ptype))
+	{
+		appendStringInfoString(buffer, "NULL");
+		return;
+	}
+
+	getTypeOutputInfo(param->ptype, &typoutput, &typisvarlena);
+	pstring = OidOutputFunctionCall(typoutput, param->value);
+	appendStringInfo(buffer, "%s", pstring);
+}
+
+/* denormalize the query, replace placeholders with actual values */
+StringInfoData
+get_denormalized_query(const ParamListInfo paramlist, const char *query_text)
+{
+	int			current_param;
+	const char *cursor_ori;
+	const char *cursor_curr;
+	StringInfoData result_buf;
+	ptrdiff_t	len;
+
+	current_param = 0;
+	cursor_ori = query_text;
+	cursor_curr = cursor_ori;
+
+	initStringInfo(&result_buf);
+
+	do
+	{
+		/* advance cursor until detecting a placeholder '$' start. */
+		while (*cursor_ori && *cursor_ori != '$')
+			++cursor_ori;
+
+		/* calculate length of query text before placeholder. */
+		len = cursor_ori - cursor_curr;
+
+		/* check if end of string is reached */
+		if (!*cursor_ori)
+		{
+			/* there may have remaining query data to append */
+			if (len > 0)
+				appendBinaryStringInfo(&result_buf, cursor_curr, len);
+
+			break;
+		}
+
+		/* append query text before the '$' sign found. */
+		if (len > 0)
+			appendBinaryStringInfo(&result_buf, cursor_curr, len);
+
+		/* skip '$' */
+		++cursor_ori;
+
+		/* skip the placeholder */
+		while (*cursor_ori && *cursor_ori >= '0' && *cursor_ori <= '9')
+			cursor_ori++;
+
+		/* advance current cursor */
+		cursor_curr = cursor_ori;
+
+		/* replace the placeholder with actual value */
+		get_param_value(paramlist, current_param, &result_buf);
+
+		++current_param;
+	} while (*cursor_ori != '\0');
+
+
+	return result_buf;
+}