Explain what a SSTable Attached Secondary Index (SASI) is.
In Cassandra 3.4 and later, SSTable Attached Secondary Indexes (SASI) have been introduced that
improve on the existing secondary index implementation with superior performance for queries that
previously required the use of
ALLOW FILTERING. SASI is significantly less
resource intensive, using less memory, disk, and CPU. It enables querying with prefix and
contains on strings, similar to the SQL implementation of
LIKE = "foo%" or
LIKE = "%foo%", as shown in SELECT. It also supports SPARSE indexing to improve
performance of querying large, dense number ranges such as time series data.
SASI takes advantage of Cassandra's write-once immutable ordered data model to build indexes when data is flushed from the memtable to disk. The SASI index data structures are built in memory as the SSTable is written and flushed to disk as sequential writes before the SSTable writing completes. One index file is written for each indexed column.
SASI supports all queries already supported by CQL, and supports the
ALLOW FILTERING is used, SASI also supports queries with multiple predicates
AND. With SASI, the performance pitfalls of using filtering are not
realized because the filtering is not performed even if
ALLOW FILTERING is
- SASI can reference offsets in the data file, skipping the Bloom filter and partition indexes to go directory to where data is stored.
- When SSTables are compacted, new indexes are generated automatically.
Currently, SASI does not support collections. Regular secondary indexes can be built for collections. Static columns are supported in Cassandra 3.6 and later.