OS: RHEL 7.2
PostgreSQL Version 9.6
I want toast to compress data. The average record length is around 500 Bytes in my tables. Although the columns show storage as extended, yet no compression is happening. Hence I want to modify toast_tuple_threshold to 500 bytes. Which file holds this value? And do we need to modify any other parameter?
I tried
ALTER TABLE tablename SET (TOAST_TUPLE_TARGET = 128);
Related
I tried to load the redshift table but failed on one column- The length of the data column 'column_description'is longer than the length defined in the table. Table: 65535, Data: 86555.
I tried to increase the length of column in RS table, looks like 65535 is the max length RS supports.
Do we have any alternatives to store value in Redshift?
The answer is that Redshift doesn't support anything larger and that one shouldn't store large artifacts in an analytic database. If you are using Redshift for its analytic powers to find specific artifacts (images, files, etc) then these should be stored in S3 and the object key (pointer) should be stored in redshift.
We had a table with a column with a Clob(length 1048576) that would store search text that helps with searching. When I transferred it from DB2 to Postgres in our migration, I found that it wasn't working as well. So I going to try text or varchar, but I was finding it would take much longer for the long text entries to be added to the table to the point my local wildfly window would timeout when trying to run.
What is the equilavelent of a datatpye that accepts text that I should be using in postgres to replace a Clob that was length 1048576 in DB2? It might be that I was using the right datatypes but didn't have the right corresponding size.
Use text. That is the only reasonable data type for long character strings.
oid -> creates a table pg_largeobjects and stores data in there
bytea -> if the compressed data would still exceed 2000 bytes, PostgreSQL splits variable length data types in chunks and stores them out of line in a special “TOAST table” according to https://www.cybertec-postgresql.com/en/binary-data-performance-in-postgresql/
I don't want any other table for large data I want to store them in a column in my defined table, is that possible?
It is best to avoid Large Objects.
With bytea you can prevent PostgreSQL from storing data out of line in a TOAST table by changing the column definition like
ALTER TABLE tab ALTER col SET STORAGE MAIN;
Then PostgreSQL will compress that column but keep it in the main table.
Since the block size in PostgreSQL is 8kB, and one row is always stored in a single block, that will limit the size of your table rows to somewhat under 8kB (there is a block header and other overhead).
I think that you are trying to solve a non-problem, and your request to not store large data out of line is unreasonable.
PostgreSQL 12 enables generated columns. Is there an overhead of using a generated column? If yes what is it?
Is table locked for read/write when such column is added?
I am approaching the 10 GB limit that Express has on the primary database file.
The main problem appears to be some fixed length char(500) columns that are never near that length.
I have two tables with about 2 million rows between them. These two tables add up to about 8 GB of data with the remainder being spread over another 20 tables or so. These two tables each have 2 char(500) columns.
I am testing a way to convert these columns to varchar(500) and recover the trailing spaces.
I tried this:
Alter Table Test_MAILBACKUP_RECIPIENTS
Alter Column SMTP_address varchar(500)
GO
Alter Table Test_MAILBACKUP_RECIPIENTS
Alter Column EXDN_address varchar(500)
This quickly changed the column type but obviously didn’t recover the space.
The only way I can see to do this successfully is to:
Create a new table in tempdb with the varchar(500) columns,
Copy the information into the temp table trimming off the trailing spaces,
Drop the real table,
Recreate the real table with the new varchar(500) columns,
Copy the information back.
I’m open to other ideas here as I’ll have to take my application offline while this process completes?
Another thing I’m curious about is the primary key identity column.
This table has a Primary Key field set as an identity.
I know I have to use Set Identity_Insert on to allow the records to be inserted into the table and turn it off when I’m finished.
How will recreating a table affect new records being inserted into the table after I’m finished. Or is this just “Microsoft Magic” and I don’t need to worry about it?
The problem with you initial approach was that you converted the columns to varchar but didn't trim the existing whitespace (which is maintained after the conversion), after changing the data type of the columns to you should do:
update Test_MAILBACKUP_RECIPIENTS set
SMTP_address=rtrim(SMTP_address), EXDN_address=rtrim(EXDN_address)
This will eliminate all trailing spaces from you table, but note that the actual disk size will be the same, as SQL Server don't shrink automatically database files, it just mark that space as unused and available for other data.
You can use this script from another question to see the actual space used by data in the DB files:
Get size of all tables in database
Usually shrinking a database is not recommended but when there is a lot of difference between used space and disk size you can do it with dbcc shrinkdatabase:
dbcc shrinkdatabase (YourDatabase, 10) -- leaving 10% of free space for new data
OK I did a SQL backup, disabled the application and tried my script anyway.
I was shocked that it ran in under 2 minutes on my slow old server.
I re-enabled my application and it still works. (Yay)
Looking at the reported size of the table now it went from 1.4GB to 126Mb! So at least that has bought me some time.
(I have circled the Data size in KB)
Before
After
My next problem is the MailBackup table which also has two char(500) columns.
It is shown as 6.7GB.
I can't use the same approach as this table contains a FileStream column which has around 190gb of data and tempdb does not support FleStream as far as I know.
Looks like this might be worth a new question.