pg_stat_statements: Add wal_buffers_full

wal_buffers_full tracks the number of times WAL buffers become full,
giving hints to be able to tune the GUC wal_buffers.

Up to now, this information was only available in pg_stat_wal.  With
this field available in WalUsage since eaf502747b, exposing it in
pg_stat_statements is straight-forward, and it offers more granularity
at query level.

pg_stat_statements does not need a version bump as one has been done in
commit cf54a2c002 for this development cycle.

Author: Bertrand Drouvot
Reviewed-by: Ilia Evdokimov
Discussion: https://postgr.es/m/Z6SOha5YFFgvpwQY@ip-10-97-1-34.eu-west-3.compute.internal

contrib/pg_stat_statements/expected/oldextversions.out

@@ -385,6 +385,7 @@ AlTER EXTENSION pg_stat_statements UPDATE TO '1.12';
  wal_records                | bigint                   |           |          | 
  wal_fpi                    | bigint                   |           |          | 
  wal_bytes                  | numeric                  |           |          | 
+ wal_buffers_full           | bigint                   |           |          | 
  jit_functions              | bigint                   |           |          | 
  jit_generation_time        | double precision         |           |          | 
  jit_inlining_count         | bigint                   |           |          | 

contrib/pg_stat_statements/pg_stat_statements--1.11--1.12.sql

@@ -50,6 +50,7 @@ CREATE FUNCTION pg_stat_statements(IN showtext boolean,
     OUT wal_records int8,
     OUT wal_fpi int8,
     OUT wal_bytes numeric,
+    OUT wal_buffers_full int8,
     OUT jit_functions int8,
     OUT jit_generation_time float8,
     OUT jit_inlining_count int8,

contrib/pg_stat_statements/pg_stat_statements.c

@@ -187,6 +187,7 @@ typedef struct Counters
 	int64		wal_records;	/* # of WAL records generated */
 	int64		wal_fpi;		/* # of WAL full page images generated */
 	uint64		wal_bytes;		/* total amount of WAL generated in bytes */
+	int64		wal_buffers_full;	/* # of times the WAL buffers became full */
 	int64		jit_functions;	/* total number of JIT functions emitted */
 	double		jit_generation_time;	/* total time to generate jit code */
 	int64		jit_inlining_count; /* number of times inlining time has been
@@ -1465,6 +1466,7 @@ pgss_store(const char *query, uint64 queryId,
 		entry->counters.wal_records += walusage->wal_records;
 		entry->counters.wal_fpi += walusage->wal_fpi;
 		entry->counters.wal_bytes += walusage->wal_bytes;
+		entry->counters.wal_buffers_full += walusage->wal_buffers_full;
 		if (jitusage)
 		{
 			entry->counters.jit_functions += jitusage->created_functions;
@@ -1557,8 +1559,8 @@ pg_stat_statements_reset(PG_FUNCTION_ARGS)
 #define PG_STAT_STATEMENTS_COLS_V1_9	33
 #define PG_STAT_STATEMENTS_COLS_V1_10	43
 #define PG_STAT_STATEMENTS_COLS_V1_11	49
-#define PG_STAT_STATEMENTS_COLS_V1_12	51
+#define PG_STAT_STATEMENTS_COLS_V1_12	52
-#define PG_STAT_STATEMENTS_COLS			51	/* maximum of above */
+#define PG_STAT_STATEMENTS_COLS			52	/* maximum of above */
 
 /*
  * Retrieve statement statistics.
@@ -1955,6 +1957,10 @@ pg_stat_statements_internal(FunctionCallInfo fcinfo,
 											Int32GetDatum(-1));
 			values[i++] = wal_bytes;
 		}
+		if (api_version >= PGSS_V1_12)
+		{
+			values[i++] = Int64GetDatumFast(tmp.wal_buffers_full);
+		}
 		if (api_version >= PGSS_V1_10)
 		{
 			values[i++] = Int64GetDatumFast(tmp.jit_functions);

doc/src/sgml/pgstatstatements.sgml

@@ -436,6 +436,15 @@
       </para></entry>
      </row>
 
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>wal_buffers_full</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Number of times the WAL buffers became full
+      </para></entry>
+     </row>
+
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>jit_functions</structfield> <type>bigint</type>