Use one stringbuffer for all rows printed in printtup.c.

This avoids newly allocating, and then possibly growing, the
stringbuffer for every row. For wide rows this can substantially
reduce memory allocator overhead, at the price of not immediately
reducing memory usage after outputting an especially wide row.

Author: Andres Freund
Discussion: https://postgr.es/m/20170914063418.sckdzgjfrsbekae4@alap3.anarazel.de

Author: anarazel

src/backend/access/common/printtup.c

@@ -57,6 +57,7 @@ typedef struct
 typedef struct
 {
 	DestReceiver pub;			/* publicly-known function pointers */
+	StringInfoData buf;			/* output buffer */
 	Portal		portal;			/* the Portal we are printing from */
 	bool		sendDescrip;	/* send RowDescription at startup? */
 	TupleDesc	attrinfo;		/* The attr info we are set up for */
@@ -127,6 +128,9 @@ printtup_startup(DestReceiver *self, int operation, TupleDesc typeinfo)
 	DR_printtup *myState = (DR_printtup *) self;
 	Portal		portal = myState->portal;
 
+	/* create buffer to be used for all messages */
+	initStringInfo(&myState->buf);
+
 	/*
 	 * Create a temporary memory context that we can reset once per row to
 	 * recover palloc'd memory.  This avoids any problems with leaks inside
@@ -302,7 +306,7 @@ printtup(TupleTableSlot *slot, DestReceiver *self)
 	TupleDesc	typeinfo = slot->tts_tupleDescriptor;
 	DR_printtup *myState = (DR_printtup *) self;
 	MemoryContext oldcontext;
-	StringInfoData buf;
+	StringInfo	buf = &myState->buf;
 	int			natts = typeinfo->natts;
 	int			i;
 
@@ -319,9 +323,9 @@ printtup(TupleTableSlot *slot, DestReceiver *self)
 	/*
 	 * Prepare a DataRow message (note buffer is in per-row context)
 	 */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage_reuse(buf, 'D');
 
-	pq_sendint(&buf, natts, 2);
+	pq_sendint(buf, natts, 2);
 
 	/*
 	 * send the attributes of this tuple
@@ -333,7 +337,7 @@ printtup(TupleTableSlot *slot, DestReceiver *self)
 
 		if (slot->tts_isnull[i])
 		{
-			pq_sendint(&buf, -1, 4);
+			pq_sendint(buf, -1, 4);
 			continue;
 		}
 
@@ -354,21 +358,21 @@ printtup(TupleTableSlot *slot, DestReceiver *self)
 			char	   *outputstr;
 
 			outputstr = OutputFunctionCall(&thisState->finfo, attr);
-			pq_sendcountedtext(&buf, outputstr, strlen(outputstr), false);
+			pq_sendcountedtext(buf, outputstr, strlen(outputstr), false);
 		}
 		else
 		{
 			/* Binary output */
 			bytea	   *outputbytes;
 
 			outputbytes = SendFunctionCall(&thisState->finfo, attr);
-			pq_sendint(&buf, VARSIZE(outputbytes) - VARHDRSZ, 4);
-			pq_sendbytes(&buf, VARDATA(outputbytes),
+			pq_sendint(buf, VARSIZE(outputbytes) - VARHDRSZ, 4);
+			pq_sendbytes(buf, VARDATA(outputbytes),
 						 VARSIZE(outputbytes) - VARHDRSZ);
 		}
 	}
 
-	pq_endmessage(&buf);
+	pq_endmessage_reuse(buf);
 
 	/* Return to caller's context, and flush row's temporary memory */
 	MemoryContextSwitchTo(oldcontext);
@@ -387,7 +391,7 @@ printtup_20(TupleTableSlot *slot, DestReceiver *self)
 	TupleDesc	typeinfo = slot->tts_tupleDescriptor;
 	DR_printtup *myState = (DR_printtup *) self;
 	MemoryContext oldcontext;
-	StringInfoData buf;
+	StringInfo	buf = &myState->buf;
 	int			natts = typeinfo->natts;
 	int			i,
 				j,
@@ -406,7 +410,7 @@ printtup_20(TupleTableSlot *slot, DestReceiver *self)
 	/*
 	 * tell the frontend to expect new tuple data (in ASCII style)
 	 */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage_reuse(buf, 'D');
 
 	/*
 	 * send a bitmap of which attributes are not null
@@ -420,13 +424,13 @@ printtup_20(TupleTableSlot *slot, DestReceiver *self)
 		k >>= 1;
 		if (k == 0)				/* end of byte? */
 		{
-			pq_sendint(&buf, j, 1);
+			pq_sendint(buf, j, 1);
 			j = 0;
 			k = 1 << 7;
 		}
 	}
 	if (k != (1 << 7))			/* flush last partial byte */
-		pq_sendint(&buf, j, 1);
+		pq_sendint(buf, j, 1);
 
 	/*
 	 * send the attributes of this tuple
@@ -443,10 +447,10 @@ printtup_20(TupleTableSlot *slot, DestReceiver *self)
 		Assert(thisState->format == 0);
 
 		outputstr = OutputFunctionCall(&thisState->finfo, attr);
-		pq_sendcountedtext(&buf, outputstr, strlen(outputstr), true);
+		pq_sendcountedtext(buf, outputstr, strlen(outputstr), true);
 	}
 
-	pq_endmessage(&buf);
+	pq_endmessage_reuse(buf);
 
 	/* Return to caller's context, and flush row's temporary memory */
 	MemoryContextSwitchTo(oldcontext);
@@ -572,7 +576,7 @@ printtup_internal_20(TupleTableSlot *slot, DestReceiver *self)
 	TupleDesc	typeinfo = slot->tts_tupleDescriptor;
 	DR_printtup *myState = (DR_printtup *) self;
 	MemoryContext oldcontext;
-	StringInfoData buf;
+	StringInfo	buf = &myState->buf;
 	int			natts = typeinfo->natts;
 	int			i,
 				j,
@@ -591,7 +595,7 @@ printtup_internal_20(TupleTableSlot *slot, DestReceiver *self)
 	/*
 	 * tell the frontend to expect new tuple data (in binary style)
 	 */
-	pq_beginmessage(&buf, 'B');
+	pq_beginmessage_reuse(buf, 'B');
 
 	/*
 	 * send a bitmap of which attributes are not null
@@ -605,13 +609,13 @@ printtup_internal_20(TupleTableSlot *slot, DestReceiver *self)
 		k >>= 1;
 		if (k == 0)				/* end of byte? */
 		{
-			pq_sendint(&buf, j, 1);
+			pq_sendint(buf, j, 1);
 			j = 0;
 			k = 1 << 7;
 		}
 	}
 	if (k != (1 << 7))			/* flush last partial byte */
-		pq_sendint(&buf, j, 1);
+		pq_sendint(buf, j, 1);
 
 	/*
 	 * send the attributes of this tuple
@@ -628,12 +632,12 @@ printtup_internal_20(TupleTableSlot *slot, DestReceiver *self)
 		Assert(thisState->format == 1);
 
 		outputbytes = SendFunctionCall(&thisState->finfo, attr);
-		pq_sendint(&buf, VARSIZE(outputbytes) - VARHDRSZ, 4);
-		pq_sendbytes(&buf, VARDATA(outputbytes),
+		pq_sendint(buf, VARSIZE(outputbytes) - VARHDRSZ, 4);
+		pq_sendbytes(buf, VARDATA(outputbytes),
 					 VARSIZE(outputbytes) - VARHDRSZ);
 	}
 
-	pq_endmessage(&buf);
+	pq_endmessage_reuse(buf);
 
 	/* Return to caller's context, and flush row's temporary memory */
 	MemoryContextSwitchTo(oldcontext);