Compressed heap bytes larger than uncompressed

debugger22m commented 4 years ago

Hi.

So, I'm really impressed by the compression ratios. However, some of the compressed chunks size are larger than uncompressed heap bytes. What could be the explanation behind it. I do have more than 5 tables in my segmentby.

EDIT: 5 columns in segmentby, NOT tables.

Thank you! Prat

mfreed commented 4 years ago

I'm not sure I'm fully following what you are saying. Is it that, for certain chunks, you see the uncompressed size less than the compressed size? (I can't parse the above given "chunks sizes vs. heap bytes).

Also, when you say "more than 5 tables in your segmentby", do you mean 5 columns?

If you specify a large number of columns, or a column with a large number of distinct items, in your segmentby, it can be that basically there is no "batching" that can occur when converting rows to columns. Basically, you only have a single (or small number) of items that match that unique segmentby (particularly within each chunk), such that the "compressed array" only has a very small number of items and thus doesn't compress well at all.

debugger22m commented 4 years ago

@mfreed

Sorry, I should have provided more details. This is for the entire hypertable

SELECT hypertable_name, uncompressed_heap_bytes, compressed_heap_bytes FROM timescaledb_information.compressed_hypertable_stats;

Yes, I meant segmentby has 5 and more columns. I will try this again by limiting the columns.

Thank you!.

NunoFilipeSantos commented 3 years ago

Hi @120bits is this still affecting you?

maggusf commented 3 years ago

having more or less the same problem. because of the bigger heap I can't compress certain table as the compressed chunk hits the error: SELECT compress_chunk(i) from show_chunks('mytable', older_than => INTERVAL '2 days') i [54000]: ERROR: row is too big: size 8648, maximum size 8160

mfundul commented 3 years ago

There may be too many columns and it hits the block size limit after inefficient compression. Can you give some more information about the schema?

maggusf commented 3 years ago

sure, here we go:

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    CONSTRAINT mon_get_database_pkey PRIMARY KEY (snapshot_time, hostname, inst_name, db_name, member)
);

mkindahl commented 3 years ago

A possible fix here would be to check the row size when turning on the compression flag rather than when trying to compress the chunk. This would give an error when compression is enabled for the table rather than when an attempt is made to compress the table, which is probably easier to follow, understand, and handle.

timescale / timescaledb

Compressed heap bytes larger than uncompressed #1784