Hi,
A while ago when blogging about WAL [1], I noticed a function to deal with
xlog location arithmetic is wanted. I remembered Depez [2] mentioning it and
after some questions during trainings and conferences I decided to translate
my shell script function in C.
The attached patch implements the function pg_xlog_location_diff (bikeshed
colors are welcome). It calculates the difference between two given
transaction log locations. Now that we have pg_stat_replication view, it will
be easy to get the lag just passing columns as parameters. Also, the
monitoring tools could take advantage of it instead of relying on a fragile
routine to get the lag.
I noticed that pg_xlogfile_name* functions does not sanity check the xrecoff
boundaries but that is material for another patch.
[1] http://eulerto.blogspot.com/2011/11/understanding-wal-nomenclature.html
[2]
http://www.depesz.com/index.php/2011/01/24/waiting-for-9-1-pg_stat_replication/
--
Euler Taveira de Oliveira - Timbira http://www.timbira.com.br/
PostgreSQL: Consultoria, Desenvolvimento, Suporte 24x7 e Treinamento
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index ddfb29a..cce218a 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -14342,6 +14342,9 @@ SELECT set_config('log_statement_stats', 'off', false);
<indexterm>
<primary>pg_xlogfile_name_offset</primary>
</indexterm>
+ <indexterm>
+ <primary>pg_xlog_location_diff</primary>
+ </indexterm>
<para>
The functions shown in <xref
@@ -14414,6 +14417,13 @@ SELECT set_config('log_statement_stats', 'off', false);
<entry><type>text</>, <type>integer</></entry>
<entry>Convert transaction log location string to file name and decimal byte offset within file</entry>
</row>
+ <row>
+ <entry>
+ <literal><function>pg_xlog_location_diff(<parameter>location</> <type>text</>, <parameter>location</> <type>text</>)</function></literal>
+ </entry>
+ <entry><type>bigint</></entry>
+ <entry>Calculate the difference between two transaction log locations</entry>
+ </row>
</tbody>
</tgroup>
</table>
@@ -14507,6 +14517,13 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
</para>
<para>
+ <function>pg_xlog_location_diff</> calculates the difference in bytes
+ between two transaction log locations. It can be used with
+ <structname>pg_stat_replication</structname> or some functions shown in
+ <xref linkend="functions-admin-backup-table"> to get the replication lag.
+ </para>
+
+ <para>
For details about proper usage of these functions, see
<xref linkend="continuous-archiving">.
</para>
diff --git a/src/backend/access/transam/xlogfuncs.c b/src/backend/access/transam/xlogfuncs.c
index 22c6ca0..09e8369 100644
--- a/src/backend/access/transam/xlogfuncs.c
+++ b/src/backend/access/transam/xlogfuncs.c
@@ -26,6 +26,7 @@
#include "replication/walreceiver.h"
#include "storage/smgr.h"
#include "utils/builtins.h"
+#include "utils/int8.h"
#include "utils/guc.h"
#include "utils/timestamp.h"
@@ -465,3 +466,57 @@ pg_is_in_recovery(PG_FUNCTION_ARGS)
{
PG_RETURN_BOOL(RecoveryInProgress());
}
+
+/*
+ * Compute the difference in bytes between two WAL locations.
+ */
+Datum
+pg_xlog_location_diff(PG_FUNCTION_ARGS)
+{
+ text *location1 = PG_GETARG_TEXT_P(0);
+ text *location2 = PG_GETARG_TEXT_P(1);
+ char *str1, *str2;
+ uint32 xlogid1, xrecoff1;
+ uint32 xlogid2, xrecoff2;
+ int64 tmp;
+ int64 result;
+
+ /*
+ * Read and parse input
+ */
+ str1 = text_to_cstring(location1);
+ str2 = text_to_cstring(location2);
+
+ if (sscanf(str1, "%8X/%8X", &xlogid1, &xrecoff1) != 2)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("could not parse transaction log location \"%s\"", str1)));
+ if (sscanf(str2, "%8X/%8X", &xlogid2, &xrecoff2) != 2)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("could not parse transaction log location \"%s\"", str2)));
+
+ /*
+ * Sanity check
+ */
+ if (xrecoff1 > XLogFileSize)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("xrecoff \"%X\" is out of valid range, 0..%X", xrecoff1, XLogFileSize)));
+ if (xrecoff2 > XLogFileSize)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("xrecoff \"%X\" is out of valid range, 0..%X", xrecoff2, XLogFileSize)));
+
+ /*
+ * Use the int8 functions mainly for overflow detection
+ *
+ * result = XLogFileSize * (xlogid1 - xlogid2) + xrecoff1 - xrecoff2
+ */
+ tmp = DirectFunctionCall2(int8mi, xlogid1, xlogid2);
+ tmp = DirectFunctionCall2(int8mul, XLogFileSize, tmp);
+ tmp = DirectFunctionCall2(int8pl, tmp, xrecoff1);
+ result = DirectFunctionCall2(int8mi, tmp, xrecoff2);
+
+ PG_RETURN_INT64(result);
+}
diff --git a/src/include/access/xlog_internal.h b/src/include/access/xlog_internal.h
index cb43879..3e7340b 100644
--- a/src/include/access/xlog_internal.h
+++ b/src/include/access/xlog_internal.h
@@ -279,5 +279,6 @@ extern Datum pg_is_in_recovery(PG_FUNCTION_ARGS);
extern Datum pg_xlog_replay_pause(PG_FUNCTION_ARGS);
extern Datum pg_xlog_replay_resume(PG_FUNCTION_ARGS);
extern Datum pg_is_xlog_replay_paused(PG_FUNCTION_ARGS);
+extern Datum pg_xlog_location_diff(PG_FUNCTION_ARGS);
#endif /* XLOG_INTERNAL_H */
diff --git a/src/include/catalog/pg_proc.h b/src/include/catalog/pg_proc.h
index 28e53b7..036d6ca 100644
--- a/src/include/catalog/pg_proc.h
+++ b/src/include/catalog/pg_proc.h
@@ -2869,6 +2869,8 @@ DATA(insert OID = 2850 ( pg_xlogfile_name_offset PGNSP PGUID 12 1 0 0 0 f f f t
DESCR("xlog filename and byte offset, given an xlog location");
DATA(insert OID = 2851 ( pg_xlogfile_name PGNSP PGUID 12 1 0 0 0 f f f t f i 1 0 25 "25" _null_ _null_ _null_ _null_ pg_xlogfile_name _null_ _null_ _null_ ));
DESCR("xlog filename, given an xlog location");
+DATA(insert OID = 3129 ( pg_xlog_location_diff PGNSP PGUID 12 1 0 0 0 f f f t f i 2 0 20 "25 25" _null_ _null_ _null_ _null_ pg_xlog_location_diff _null_ _null_ _null_ ));
+DESCR("difference in bytes, given two xlog locations");
DATA(insert OID = 3809 ( pg_export_snapshot PGNSP PGUID 12 1 0 0 0 f f f t f v 0 0 25 "" _null_ _null_ _null_ _null_ pg_export_snapshot _null_ _null_ _null_ ));
DESCR("export a snapshot");
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