Tables with fragmentation
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RStoolKit Results: Tables with fragmentation
Fragmentation in RedShift is nothing but the Ghost rows and unsorted rows. Whenever we did a delete or update the old row will be marked as deleted. When we run the VACUUM DELETE then these rows will be permanently removed. But the space removed by the vacuum will be reclaimed, but its now fragmented. Until you run the VACUUM SORT ONLY or VACUUM FULL this space will not be defragmented.
Find tables with fragmentation:
In RedShift admin views, we have a view to find the fragmentation.
-- AWS RedShift Admin view
-- https://github.com/awslabs/amazon-redshift-utils/blob/master/src/AdminViews/v_fragmentation_info.sql
SELECT tbl,
tablename,
dbname,
SUM(t_excess_blks) est_space_gain
FROM (SELECT tbl,
col,
node,
tablename,
Trim(datname) AS dbname,
SUM(excess_blks) * ( col + 1 ) AS t_excess_blks
FROM (SELECT tbl,
slice,
col,
Count(*) total_blks
FROM stv_blocklist
WHERE num_values > 0
GROUP BY 1,
2,
3) a
join (SELECT tbl,
slice,
Max(col) AS col
FROM stv_blocklist
GROUP BY 1,
2) b USING (tbl, slice, col)
join (SELECT tbl,
slice,
col,
Count(*) - Ceil(SUM(num_values) / 130994.0) AS
excess_blks
FROM stv_blocklist
WHERE num_values > 0
AND num_values < 130994
GROUP BY 1,
2,
3) c USING (tbl, slice, col)
join stv_slices d USING (slice)
join (SELECT id,
Trim("name") AS tablename,
db_id
FROM stv_tbl_perm
WHERE slice = 0) f
ON b.tbl = f.id
join pg_database g
ON f.db_id = g.oid
WHERE excess_blks > 1
GROUP BY 1,
2,
3,
4,
5)
WHERE tbl > 1
AND t_excess_blks > (SELECT CASE
WHEN SUM(capacity) > 200000 THEN 1024
ELSE 102.4
END
FROM stv_partitions
WHERE host = owner
AND host = 0
GROUP BY host)
GROUP BY 1,
2,
3
ORDER BY 4 DESC;
How to fix this problem:
To fix the table’s fragmentation we need to run the Vacuum to reclaim the space.
Vacuum Full - permanently delete the ghost rows, reclaim the space and sort them all.
VACUUM [ FULL | SORT ONLY | DELETE ONLY | REINDEX ]
[ [ table_name ] [ TO threshold PERCENT ] [ BOOST ] ]
Also don’t forget to run the analyze.
External Links:
- RedShift vacuum
- Get the ghost rows for all the tables
- RedShift tombstone block
- RedShift analyze
- RedShift vacuum Utility - Python Based
- Automate Vacuum Analyze Utility - Shell based with more control
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