Re: Significantly larger toast tables on 8.4? - Mailing list pgsql-hackers

On Fri, Jan 2, 2009 at 09:01, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>The argument was that compressing/decompressing such large chunks would
>require a lot of CPU effort; also it would defeat attempts to fetch
>subsections of a large string.  In the past we've required people to
>explicitly "ALTER TABLE SET STORAGE external" if they wanted to make
>use of the substring-fetch optimization, but it was argued that this
>would make that more likely to work automatically.

It is *way* faster  here are some ruff numbers:
$ cat q.sql
select length(substring(debug, 1024, 1024)) from dies where die_id = 295;

$ ./pgbench -T60 -n -f q.sql
8.4: 1532.327546
8.3:      21.295657

Thats with 8.4 as a  --enable-debug --enable-cassert build! (8.3 just
has --enable-debug)

> I'm not entirely convinced by Alex' analysis anyway; the only way
> those 39 large values explain the size difference is if they are
> *tremendously* compressible, like almost all zeroes.  The toast
> compressor isn't so bright that it's likely to get 10X compression
> on typical data.

Well its certainly not all zeros, but those big rows are the same
message repeated (~5k) until perl detected endless recursion died...
So its not exactly typical or *useful* data either.   The other 51,
971 rows (rows smaller than 1024*1024 bytes) account for 174MB while
those 39 account for the other 828MB.

Mainly I was just alarmed by the size increase.  But I think the
pgbench numbers make me happy even if I never do a substring on that
data...

------------------
# 8.3
select pg_size_pretty(pg_column_size(debug)) from dies where die_id = 295;pg_size_pretty
----------------1256 kB

select pg_size_pretty(pg_column_size(debug)) from dies where die_id = 295;pg_size_pretty
----------------22 MB
select pg_size_pretty(sum(row_size)) from
( select      coalesce(pg_column_size(action), 0) +      coalesce(pg_column_size(die_id), 0) +
coalesce(pg_column_size(cparam),0) +      coalesce(pg_column_size(date_created), 0) +
coalesce(pg_column_size(db_profile),0) +      coalesce(pg_column_size(debug), 0) +
coalesce(pg_column_size(defunct),0) +      coalesce(pg_column_size(env), 0) +      coalesce(pg_column_size(login), 0) +
    coalesce(pg_column_size(msg), 0) +      coalesce(pg_column_size(open_user_id), 0) +
coalesce(pg_column_size(page_load_id),0) +      coalesce(pg_column_size(session_id), 0) +
coalesce(pg_column_size(state),0) +      coalesce(pg_column_size(state_action), 0) +
coalesce(pg_column_size(user_id),0) +      coalesce(pg_column_size(whoops), 0) as row_sizefrom dies ) as foo where
foo.row_size< 1024*1024;pg_size_pretty
 
----------------174 MB
select pg_size_pretty(sum(row_size)) from
( select      coalesce(pg_column_size(action), 0) +      coalesce(pg_column_size(die_id), 0) +
coalesce(pg_column_size(cparam),0) +      coalesce(pg_column_size(date_created), 0) +
coalesce(pg_column_size(db_profile),0) +      coalesce(pg_column_size(debug), 0) +
coalesce(pg_column_size(defunct),0) +      coalesce(pg_column_size(env), 0) +      coalesce(pg_column_size(login), 0) +
    coalesce(pg_column_size(msg), 0) +      coalesce(pg_column_size(open_user_id), 0) +
coalesce(pg_column_size(page_load_id),0) +      coalesce(pg_column_size(session_id), 0) +
coalesce(pg_column_size(state),0) +      coalesce(pg_column_size(state_action), 0) +
coalesce(pg_column_size(user_id),0) +      coalesce(pg_column_size(whoops), 0) as row_sizefrom dies ) as foo where
foo.row_size< 1024*1024;pg_size_pretty
 
----------------828 MB