Add new function ptrack_get_change_file_stat(start_lsn pg_lsn)

Author: ololobus

Diff for: README.md

@@ -65,7 +65,9 @@ To disable `ptrack` and clean up all remaining service files set `ptrack.map_siz
 
  * ptrack_version() — returns ptrack version string.
  * ptrack_init_lsn() — returns LSN of the last ptrack map initialization.
- * ptrack_get_pagemapset('LSN') — returns a set of changed data files with bitmaps of changed blocks since specified LSN.
+ * ptrack_get_pagemapset(start_lsn pg_lsn) — returns a set of changed data files with bitmaps of changed blocks since specified `start_lsn`.
+ * ptrack_get_change_stat(start_lsn pg_lsn) — returns statistic of changes (number of files, pages and size in MB) since specified `start_lsn`.
+ * ptrack_get_change_file_stat(start_lsn pg_lsn) — returns per file statistic of changes (number of pages and size in MB) since specified `start_lsn`.
 
 Usage example:
 
@@ -102,6 +104,10 @@ Usually, you have to only install new version of `ptrack` and do `ALTER EXTENSIO
 * Do `ALTER EXTENSION 'ptrack' UPDATE;`.
 * Restart your server.
 
+#### Upgrading from 2.1.* to 2.2.*:
+
+Since version 2.2 we use a different algorithm for tracking changed pages. Thus, data recorded in the `ptrack.map` using pre 2.2 versions of `ptrack` is incompatible with newer versions. After extension upgrade and server restart old `ptrack.map` will be discarded with `WARNING` and initialized from the scratch.
+
 ## Limitations
 
 1. You can only use `ptrack` safely with `wal_level >= 'replica'`. Otherwise, you can lose tracking of some changes if crash-recovery occurs, since [certain commands are designed not to write WAL at all if wal_level is minimal](https://www.postgresql.org/docs/12/populate.html#POPULATE-PITR), but we only durably flush `ptrack` map at checkpoint time.

Diff for: ptrack--2.1--2.2.sql

@@ -27,3 +27,28 @@ BEGIN
 		 FROM ptrack_get_pagemapset(start_lsn)) s;
 END
 $func$ LANGUAGE plpgsql;
+
+CREATE FUNCTION ptrack_get_change_file_stat(start_lsn pg_lsn)
+	RETURNS TABLE (
+		file_path text,
+		pages int,
+		"size, MB" numeric
+	) AS
+$func$
+DECLARE
+block_size bigint;
+BEGIN
+	block_size := (SELECT setting FROM pg_settings WHERE name = 'block_size');
+
+	RETURN QUERY
+	SELECT s.path,
+		   changed_pages,
+		   block_size*changed_pages/(1024.0*1024)
+	FROM
+		(SELECT path,
+				length(replace(right((pagemap)::text, -1)::varbit::text, '0', ''))
+					AS changed_pages
+		 FROM ptrack_get_pagemapset(start_lsn)) s
+	ORDER BY (changed_pages, s.path) DESC;
+END
+$func$ LANGUAGE plpgsql;

Diff for: t/001_basic.pl

@@ -10,7 +10,7 @@
 use TestLib;
 use Test::More;
 
-plan tests => 24;
+plan tests => 25;
 
 my $node;
 my $res;
@@ -119,6 +119,9 @@
 $res_stdout = $node->safe_psql("postgres", "SELECT pages FROM ptrack_get_change_stat('$flush_lsn')");
 is($res_stdout > 0, 1, 'should be able to get aggregated stats of changes');
 
+$res_stdout = $node->safe_psql("postgres", "SELECT count(*) FROM ptrack_get_change_file_stat('$flush_lsn')");
+is($res_stdout > 0, 1, 'should be able to get per file stats of changes');
+
 # We should be able to change ptrack map size (but loose all changes)
 $node->append_conf(
 	'postgresql.conf', q{