From ae66670c99e9c4b0a0c918ca8947b35263379713 Mon Sep 17 00:00:00 2001
From: Tyler Fontaine <tyler@tylerfontaine.com>
Date: Fri, 9 Jul 2021 12:09:44 -0500
Subject: [PATCH] Add Procedure Allowing DDL changes to Compressed Hypertables
 via Migration

To avoid having to decompress an entire hypertable to make a DDL change
this procedure facilitates copying data to a new table with the correct
structure. It copies whole chunks at a time, and compresses them in the
new table as it goes.
---
 README.md                        |  1 +
 utils/migrate_compressed_ddl.sql | 81 ++++++++++++++++++++++++++++++++
 2 files changed, 82 insertions(+)
 create mode 100644 utils/migrate_compressed_ddl.sql

diff --git a/README.md b/README.md
index 516c775..7ccb502 100644
--- a/README.md
+++ b/README.md
@@ -7,6 +7,7 @@ core database, including those written in PL/pgSQL.
 The current list of "extras" include:
 
 - Procedures to help backfill data into compressed ranges
+- Procedure to help make DDL changes to Compressed Tables via migration to new table
 
 ## Useful views
 
diff --git a/utils/migrate_compressed_ddl.sql b/utils/migrate_compressed_ddl.sql
new file mode 100644
index 0000000..686d259
--- /dev/null
+++ b/utils/migrate_compressed_ddl.sql
@@ -0,0 +1,81 @@
+-- This utility is for helping to make major DDL changes to compressed 
+-- hypertables. Because you must decompress the entire hypertable, one
+-- might not have sufficient space to do that. An alternate method is to
+-- copy the data from one table to a new table with the correct schema
+-- or constraints, etc. This tool helps you do that chunk by chunk,
+-- compressing as you go to greatly reduce the space requirements to
+-- perform the migration.
+--
+-- Prerequisites:
+--   The target table must be created first, and compression must be
+--   configured for it.
+--    
+--   You must have sufficient space for the compressed data, so roughly
+--   double your current hypertable size, which is usually still
+--   considerably smaller than all the uncompressed data.
+--
+-- Variables:
+--   old_table is the original table to copy from
+--   new_table is the table you are copying to
+--   older_than is the limit for the chunks to recompress. It will
+--   recompress all chunks older than the interval specified
+--
+-- Limitations:
+--   This tool does not re-create policies, continuous aggregates, or
+--   other dependent objects. It only copies the data to a new table.
+--
+-- Usage:
+--   CALL migrate_data('copy_from', 'copy_to', '30 days');
+--
+ 
+
+
+CREATE OR REPLACE procedure migrate_data(
+	old_table text, 
+	new_table text, 
+	older_than interval)
+language plpgsql
+as 
+$$
+DECLARE
+c_row record;
+c_curs cursor for 
+select * from chunknames;
+last_chunk text;
+
+BEGIN
+
+--create temp table for storing original hypertable chunk names
+CREATE TEMPORARY TABLE chunknames (chunkname text);
+
+insert into chunknames
+(select * from show_chunks(old_table));
+
+-- loop through (one at a time) the chunks and copy the data to the new 
+-- hypertable
+FOR c_row in c_curs LOOP
+	EXECUTE format('insert into %I
+	(select * from %s)', new_table, c_row.chunkname);
+	
+	-- get most recent chunk from the new hypertable after copying the 
+	-- whole other chunk is finished.
+	-- it will only grab chunks to drop up to the older_than interval 
+	-- specified.
+	select a.show_chunks into last_chunk from 
+	(select * from 
+	show_chunks('copyto', older_than => older_than)
+    order by show_chunks DESC
+    LIMIT 1) a;
+	
+	RAISE NOTICE 'Copied Chunk % into %', t_row.chunkname, last_chunk;
+	
+	-- compress that last chunk. 
+	PERFORM compress_chunk(last_chunk);
+	
+	RAISE NOTICE 'Compressed %', last_chunk;
+	END LOOP;
+	
+
+drop table chunknames;
+END;
+$$;