Speed up creation of command completion tags

The building of command completion tags could often be seen showing up in
profiles when running high tps workloads.

The query completion tags were being built with snprintf, which is slow at
the best of times when compared with more manual ways of formatting
strings.  Here we introduce BuildQueryCompletionString() to do this job
for us.  We also now store the completion tag's strlen in the
CommandTagBehavior struct so that we can quickly memcpy this number of
bytes into the completion tag string.  Appending the rows affected is done
via pg_ulltoa_n.  BuildQueryCompletionString returns the length of the
built string.  This saves us having to call strlen to figure out how many
bytes to pass to pq_putmessage().

Author: David Rowley, Andres Freund
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/CAHoyFK-Xwqc-iY52shj0G+8K9FJpse+FuZ36XBKy78wDVnd=Qg@mail.gmail.com

src/backend/tcop/cmdtag.c

@@ -15,18 +15,21 @@
 
 #include "miscadmin.h"
 #include "tcop/cmdtag.h"
+#include "utils/builtins.h"
 
 
 typedef struct CommandTagBehavior
 {
-	const char *name;
+	const char *name;			/* tag name, e.g. "SELECT" */
+	const uint8 namelen;		/* set to strlen(name) */
 	const bool	event_trigger_ok;
 	const bool	table_rewrite_ok;
-	const bool	display_rowcount;
+	const bool	display_rowcount;	/* should the number of rows affected be
+									 * shown in the command completion string */
 } CommandTagBehavior;
 
 #define PG_CMDTAG(tag, name, evtrgok, rwrok, rowcnt) \
-	{ name, evtrgok, rwrok, rowcnt },
+	{ name, (uint8) (sizeof(name) - 1), evtrgok, rwrok, rowcnt },
 
 static const CommandTagBehavior tag_behavior[COMMAND_TAG_NEXTTAG] = {
 #include "tcop/cmdtaglist.h"
@@ -47,6 +50,13 @@ GetCommandTagName(CommandTag commandTag)
 	return tag_behavior[commandTag].name;
 }
 
+const char *
+GetCommandTagNameAndLen(CommandTag commandTag, Size *len)
+{
+	*len = (Size) tag_behavior[commandTag].namelen;
+	return tag_behavior[commandTag].name;
+}
+
 bool
 command_tag_display_rowcount(CommandTag commandTag)
 {
@@ -96,3 +106,59 @@ GetCommandTagEnum(const char *commandname)
 	}
 	return CMDTAG_UNKNOWN;
 }
+
+/*
+ * BuildQueryCompletionString
+ *		Build a string containing the command tag name with the
+ *		QueryCompletion's nprocessed for command tags with display_rowcount
+ *		set.  Returns the strlen of the constructed string.
+ *
+ * The caller must ensure that buff is at least COMPLETION_TAG_BUFSIZE bytes.
+ *
+ * If nameonly is true, then the constructed string will contain only the tag
+ * name.
+ */
+Size
+BuildQueryCompletionString(char *buff, const QueryCompletion *qc,
+						   bool nameonly)
+{
+	CommandTag	tag = qc->commandTag;
+	Size		taglen;
+	const char *tagname = GetCommandTagNameAndLen(tag, &taglen);
+	char	   *bufp;
+
+	/*
+	 * We assume the tagname is plain ASCII and therefore requires no encoding
+	 * conversion.
+	 */
+	memcpy(buff, tagname, taglen);
+	bufp = buff + taglen;
+
+	/* ensure that the tagname isn't long enough to overrun the buffer */
+	Assert(taglen <= COMPLETION_TAG_BUFSIZE - MAXINT8LEN - 4);
+
+	/*
+	 * In PostgreSQL versions 11 and earlier, it was possible to create a
+	 * table WITH OIDS.  When inserting into such a table, INSERT used to
+	 * include the Oid of the inserted record in the completion tag.  To
+	 * maintain compatibility in the wire protocol, we now write a "0" (for
+	 * InvalidOid) in the location where we once wrote the new record's Oid.
+	 */
+	if (command_tag_display_rowcount(tag) && !nameonly)
+	{
+		if (tag == CMDTAG_INSERT)
+		{
+			*bufp++ = ' ';
+			*bufp++ = '0';
+		}
+		*bufp++ = ' ';
+		bufp += pg_ulltoa_n(qc->nprocessed, bufp);
+	}
+
+	/* and finally, NUL terminate the string */
+	*bufp = '\0';
+
+	Assert((bufp - buff) == strlen(buff));
+
+	return bufp - buff;
+}

src/backend/tcop/dest.c

@@ -166,8 +166,7 @@ void
 EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_output)
 {
 	char		completionTag[COMPLETION_TAG_BUFSIZE];
-	CommandTag	tag;
-	const char *tagname;
+	Size		len;
 
 	switch (dest)
 	{
@@ -175,29 +174,9 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 		case DestRemoteExecute:
 		case DestRemoteSimple:
 
-			/*
-			 * We assume the tagname is plain ASCII and therefore requires no
-			 * encoding conversion.
-			 */
-			tag = qc->commandTag;
-			tagname = GetCommandTagName(tag);
-
-			/*
-			 * In PostgreSQL versions 11 and earlier, it was possible to
-			 * create a table WITH OIDS.  When inserting into such a table,
-			 * INSERT used to include the Oid of the inserted record in the
-			 * completion tag.  To maintain compatibility in the wire
-			 * protocol, we now write a "0" (for InvalidOid) in the location
-			 * where we once wrote the new record's Oid.
-			 */
-			if (command_tag_display_rowcount(tag) && !force_undecorated_output)
-				snprintf(completionTag, COMPLETION_TAG_BUFSIZE,
-						 tag == CMDTAG_INSERT ?
-						 "%s 0 " UINT64_FORMAT : "%s " UINT64_FORMAT,
-						 tagname, qc->nprocessed);
-			else
-				snprintf(completionTag, COMPLETION_TAG_BUFSIZE, "%s", tagname);
-			pq_putmessage('C', completionTag, strlen(completionTag) + 1);
+			len = BuildQueryCompletionString(completionTag, qc,
+											 force_undecorated_output);
+			pq_putmessage('C', completionTag, len + 1);
 
 		case DestNone:
 		case DestDebug:

src/include/tcop/cmdtag.h

@@ -13,6 +13,8 @@
 #ifndef CMDTAG_H
 #define CMDTAG_H
 
+/* buffer size required for command completion tags */
+#define COMPLETION_TAG_BUFSIZE	64
 
 #define PG_CMDTAG(tag, name, evtrgok, rwrok, rowcnt) \
 	tag,
@@ -50,9 +52,12 @@ CopyQueryCompletion(QueryCompletion *dst, const QueryCompletion *src)
 
 extern void InitializeQueryCompletion(QueryCompletion *qc);
 extern const char *GetCommandTagName(CommandTag commandTag);
+extern const char *GetCommandTagNameAndLen(CommandTag commandTag, Size *len);
 extern bool command_tag_display_rowcount(CommandTag commandTag);
 extern bool command_tag_event_trigger_ok(CommandTag commandTag);
 extern bool command_tag_table_rewrite_ok(CommandTag commandTag);
 extern CommandTag GetCommandTagEnum(const char *commandname);
+extern Size BuildQueryCompletionString(char *buff, const QueryCompletion *qc,
+									   bool nameonly);
 
 #endif							/* CMDTAG_H */

src/include/tcop/dest.h

@@ -71,8 +71,6 @@
 #include "tcop/cmdtag.h"
 
 
-/* buffer size to use for command completion tags */
-#define COMPLETION_TAG_BUFSIZE	64
 
 
 /* ----------------