I see. But in my case rows don't reach that thresold (I didn't check if 2K but I didn't change anything). So I'm wondering if there is any other chance except the TOAST to get the rows compressed or not.
Are you really sure you want that? For small files the overhead of compression tends to out weigh the benefits. A contrived example biased to making my point:
aklaver@killi:~> dd if=/dev/urandom of=file.txt bs=10 count=10 10+0 records in 10+0 records out 100 bytes (100 B) copied, 0.253617 s, 0.4 kB/s aklaver@killi:~> l -h file.txt -rw-r--r-- 1 aklaver users 100 Dec 15 13:07 file.txt aklaver@killi:~> gzip file.txt aklaver@killi:~> l -h file.txt.gz -rw-r--r-- 1 aklaver users 132 Dec 15 13:07 file.txt.gz
I noticed that, when I use constant data, the total IO writes (by iostat) are more or less 1/2 of the the total IO writes when using random or other data hard to compress.
Define constant data?
I thought the data you are inputting is below the compression threshold?
Is I/O causing a problem or to put it another way, what is the problem you are trying to solve?
I haven't got a problem. I'm just trying to estimate a worst case of total IO writes of my DB on the field, over the years.
If tuples get compressed, then all depends on the data compressibility.
So, either 1) I understand if/when they get compressed or 2) I must simulate with different kinds of data (compressible and not).
Furthermore, in general I like, if possible, to be aware of what is going behind the scenes.
As for 2) I made 3 tests, each inserting 1million rows of 100 bytea on my 'dati' field. First I tried 100 bytes=constant data=\\x7b7b7b7b.... then I tried 100 bytes=random and finally inserting 100 bytes taken from random offsets of a compressed file.
It seems that IO writes strongly depend on the data, even if dati field length is just 100.