This documentation is for an unreleased version of Apache Paimon. We recommend you use the latest stable version.

Overview #

If you define a table with primary key, you can insert, update or delete records in the table.

Primary keys consist of a set of columns that contain unique values for each record. Paimon enforces data ordering by sorting the primary key within each bucket, allowing users to achieve high performance by applying filtering conditions on the primary key. See CREATE TABLE.

Bucket #

Unpartitioned tables, or partitions in partitioned tables, are sub-divided into buckets, to provide extra structure to the data that may be used for more efficient querying.

Each bucket directory contains an LSM tree and its changelog files.

The range for a bucket is determined by the hash value of one or more columns in the records. Users can specify bucketing columns by providing the bucket-key option. If no bucket-key option is specified, the primary key (if defined) or the complete record will be used as the bucket key.

A bucket is the smallest storage unit for reads and writes, so the number of buckets limits the maximum processing parallelism. This number should not be too big, though, as it will result in lots of small files and low read performance. In general, the recommended data size in each bucket is about 200MB - 1GB.

Also, see rescale bucket if you want to adjust the number of buckets after a table is created.

LSM Trees #

Paimon adapts the LSM tree (log-structured merge-tree) as the data structure for file storage. This documentation briefly introduces the concepts about LSM trees.

Sorted Runs #

LSM tree organizes files into several sorted runs. A sorted run consists of one or multiple data files and each data file belongs to exactly one sorted run.

Records within a data file are sorted by their primary keys. Within a sorted run, ranges of primary keys of data files never overlap.

As you can see, different sorted runs may have overlapping primary key ranges, and may even contain the same primary key. When querying the LSM tree, all sorted runs must be combined and all records with the same primary key must be merged according to the user-specified merge engine and the timestamp of each record.

New records written into the LSM tree will be first buffered in memory. When the memory buffer is full, all records in memory will be sorted and flushed to disk. A new sorted run is now created.

Compaction #

When more and more records are written into the LSM tree, the number of sorted runs will increase. Because querying an LSM tree requires all sorted runs to be combined, too many sorted runs will result in a poor query performance, or even out of memory.

To limit the number of sorted runs, we have to merge several sorted runs into one big sorted run once in a while. This procedure is called compaction.

However, compaction is a resource intensive procedure which consumes a certain amount of CPU time and disk IO, so too frequent compaction may in turn result in slower writes. It is a trade-off between query and write performance. Paimon currently adapts a compaction strategy similar to Rocksdb’s universal compaction.

By default, when Paimon appends records to the LSM tree, it will also perform compactions as needed. Users can also choose to perform all compactions in a dedicated compaction job. See dedicated compaction job for more info.