Expose BufferUsageAccumDiff().

Previously pg_stat_statements calculated the difference of buffer counters
by its own code even while BufferUsageAccumDiff() had the same code.
This commit expose BufferUsageAccumDiff() and makes pg_stat_statements
use it for the calculation, in order to simply the code.

This change also would be useful for the upcoming patch for the planning
counters in pg_stat_statements because the patch will add one more code
for the calculation of difference of buffer counters and that can easily be
done by using BufferUsageAccumDiff().

Author: Julien Rouhaud
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/bdfee4e0-a304-2498-8da5-3cb52c0a193e@oss.nttdata.com

Author: MasaoFujii

contrib/pg_stat_statements/pg_stat_statements.c

@@ -1016,30 +1016,8 @@ pgss_ProcessUtility(PlannedStmt *pstmt, const char *queryString,
 		rows = (qc && qc->commandTag == CMDTAG_COPY) ? qc->nprocessed : 0;
 
 		/* calc differences of buffer counters. */
-		bufusage.shared_blks_hit =
-			pgBufferUsage.shared_blks_hit - bufusage_start.shared_blks_hit;
-		bufusage.shared_blks_read =
-			pgBufferUsage.shared_blks_read - bufusage_start.shared_blks_read;
-		bufusage.shared_blks_dirtied =
-			pgBufferUsage.shared_blks_dirtied - bufusage_start.shared_blks_dirtied;
-		bufusage.shared_blks_written =
-			pgBufferUsage.shared_blks_written - bufusage_start.shared_blks_written;
-		bufusage.local_blks_hit =
-			pgBufferUsage.local_blks_hit - bufusage_start.local_blks_hit;
-		bufusage.local_blks_read =
-			pgBufferUsage.local_blks_read - bufusage_start.local_blks_read;
-		bufusage.local_blks_dirtied =
-			pgBufferUsage.local_blks_dirtied - bufusage_start.local_blks_dirtied;
-		bufusage.local_blks_written =
-			pgBufferUsage.local_blks_written - bufusage_start.local_blks_written;
-		bufusage.temp_blks_read =
-			pgBufferUsage.temp_blks_read - bufusage_start.temp_blks_read;
-		bufusage.temp_blks_written =
-			pgBufferUsage.temp_blks_written - bufusage_start.temp_blks_written;
-		bufusage.blk_read_time = pgBufferUsage.blk_read_time;
-		INSTR_TIME_SUBTRACT(bufusage.blk_read_time, bufusage_start.blk_read_time);
-		bufusage.blk_write_time = pgBufferUsage.blk_write_time;
-		INSTR_TIME_SUBTRACT(bufusage.blk_write_time, bufusage_start.blk_write_time);
+		memset(&bufusage, 0, sizeof(BufferUsage));
+		BufferUsageAccumDiff(&bufusage, &pgBufferUsage, &bufusage_start);
 
 		pgss_store(queryString,
 				   0,			/* signal that it's a utility stmt */

src/backend/executor/instrument.c

@@ -21,8 +21,6 @@ BufferUsage pgBufferUsage;
 static BufferUsage save_pgBufferUsage;
 
 static void BufferUsageAdd(BufferUsage *dst, const BufferUsage *add);
-static void BufferUsageAccumDiff(BufferUsage *dst,
-								 const BufferUsage *add, const BufferUsage *sub);
 
 
 /* Allocate new instrumentation structure(s) */
@@ -203,7 +201,7 @@ BufferUsageAdd(BufferUsage *dst, const BufferUsage *add)
 }
 
 /* dst += add - sub */
-static void
+void
 BufferUsageAccumDiff(BufferUsage *dst,
 					 const BufferUsage *add,
 					 const BufferUsage *sub)

src/include/executor/instrument.h

@@ -81,5 +81,7 @@ extern void InstrAggNode(Instrumentation *dst, Instrumentation *add);
 extern void InstrStartParallelQuery(void);
 extern void InstrEndParallelQuery(BufferUsage *result);
 extern void InstrAccumParallelQuery(BufferUsage *result);
+extern void BufferUsageAccumDiff(BufferUsage *dst,
+								 const BufferUsage *add, const BufferUsage *sub);
 
 #endif							/* INSTRUMENT_H */