= SAI FAQ
:description: Frequently asked questions about Storage-Attached Indexing (SAI).

Use this FAQ to find answers to common questions and get help with Storage-Attached Indexing (SAI).

== What is SAI?

Storage-Attached Indexing (SAI) is a highly-scalable, globally-distributed index for the {cassandra} database.
SAI combines:

* the storage-attached architecture of open source SSTable Attached Secondary Indexes (SASI)
* a number of highly optimized on-disk index structures

== Which databases are supported?

include::cassandra:partial$sai/support-databases.adoc[]

After creating your database, a keyspace, and one or more tables, use `+CREATE INDEX ...
USING 'sai'+` to define one or more SAI indexes on the table.
For Cassandra databases, use `cqlsh`.
The same `+CREATE INDEX ...
USING 'sai'+` command is available for both.
See xref:cassandra:getting-started/sai-quickstart.adoc[SAI quickstart].

== What configuration settings should I use with SAI?

Compared with most indexing environments, SAI configuration and related settings are much simpler.
Key points:

* Increase `--XX:MaxDirectMemorySize`, leaving approximately 15-20% of memory for the OS and other in-memory structures.
* In cassandra.yaml, explicitly set `file_cache_size_in_mb` to 75% of that value.
* Heavy mixed read/write workloads may want to:
 ** Decrease `range_request_timeout_in_ms`
 ** Increase `write_request_timeout_in_ms`
* If the `memtable_flush_writers` value is set too low, writes may stall.
If this occurs in your environment, increase `memtable_flush_writers`.

Aside from memory, SAI uses the same tunable parameters for Cassandra, such as compaction throughput and compaction executors.
This matters for write performance.
For read performance, again, maximizing use of the Chunk Cache will benefit SAI reads because all on-disk index components are accessed through this mechanism.
Refer to xref:cassandra:developing/cql/indexing/sai/operations/configuring.adoc[Configure SAI indexes].

== What computing challenges does SAI solve?

Oftentimes, developers ask: "How can I query additional fields outside of the {product} partition key?"

SAI implements efficient indexes based on a table's columns, such as parts of a composite partition key.
Before SAI, you could index clustering keys, but you could not index parts of a composite partition.
The development of SAI was inspired by SASI to achieve the goal of efficient and simpler filtering via the creation of secondary indexes.

SAI also makes data modeling easier because you do not need to create custom tables just to cater to particular query patterns.
You can create a table that is most natural for you, write to just that table, and query it any way you want.

== What are the advantages of using SAI?

SAI makes it possible to define multiple indexes on the same database table.
Each SAI index can be based on any column in the table.
Exception: there is no need to define an SAI index based on the partition key when it's comprised of only one column;
in this case, SAI issues an `invalid query` message.
You can also define an SAI index using a single column in the table's composite partition key.
A composite partition key means that the partition is based on two or more columns.
In this case with an SAI index, you would specify just one of the columns that comprises the composite partition key.

For developers, SAI removes several previous pain points, including the need to duplicate denormalized data to query non-PrimaryKey columns.

For operators, SAI has several advantages, including the use of significantly less disk space for indexing;
fewer failure points;
easier uptime due to the simplified architecture of SAI;
and fewer copies of data to secure.

== Is SAI a complete search solution?

SAI is not an enterprise search engine.
While it does provide some of the same functionality, SAI is not a complete replacement for text-based search.
At its core, SAI is a filtering engine, and simplifies data modeling and client applications that would otherwise rely heavily on maintaining multiple query-specific tables.

== How does schema management compare between SAI and text-based search?

SAI is an index, not a search engine.
Unlike the text-based search, SAI has no need for schema management.
SAI configuration is simpler and is tuned with existing database parameters, such as in cassandra.yaml.
With SAI, there is no commit log to accept writes during bootstrap;
SAI does not need to wait for bootstrap to read the database configuration.
With SAI, schema/indexing options reside in the index metadata, which is handled by native database schema management.

== How do I use SAI features?
Storage-Attached Indexing has queries are entirely CQL-based.
The features, by design, are intentionally simple and easy to use.

At a high level, SAI indexes are:

* Created and dropped per column via CQL `+CREATE INDEX ...
USING 'sai'+` commands and `DROP INDEX` commands.
Start in xref:cassandra:getting-started/sai-quickstart.adoc[SAI quickstart].

* Rebuilt and backed up via nodetool.
See xref:managing/tools/nodetool/nodetool.adoc[nodetool].

* Monitored via a combination of nodetool, CQL virtual tables, system metrics, and JMX.
See xref:developing/cql/indexing/sai/operations/monitoring.adoc[Monitor SAI indexes].

// LLP: There are currently no limits placed on SAI indexes in Apache C*.
// [[saiLimitsFaq]]
// == Are there limits on the number of SAI indexes per table, and total per cluster?

// include::cassandra:partial$sai/config-limits.adoc[]

== On which column in a database table can I base an SAI index?

Define each SAI index on any table column.
Exception: there is no need to define an SAI index based on the partition key when it's comprised of only one column;
in this case, SAI issues an `invalid query` message.

You can also define an SAI index using a single column in the table's composite partition key.
A composite partition key means that the partition is based on two or more columns.
In this case with an SAI index, you would specify just one of the columns that comprises the composite partition key.

With collection maps, you can define one or more SAI indexes on the same column, specifying `keys`, `values`, and `entries` as map types.
SAI also supports `list` and `set` collections.

--
include::cassandra:partial$sai/collections-note.adoc[]
--

== When I `DROP` and recreate an SAI index on the same column, does that block any read operations? And is there a way to check the indexing status?


--
include::cassandra:partial$sai/queryable-paragraph.adoc[]
--

See xref:cassandra:developing/cql/indexing/sai/operations/monitoring.adoc#saiMonitorVirtualTables[Virtual tables] and xref:reference/sai-virtual-table-indexes.adoc[Virtual tables for SAI indexes and SSTables].

== What are the write and read paths used by SAI indexes?

SAI indexes Memtables and SSTables as they are written, resolving the differences between those indexes at read time.
See xref:developing/cql/indexing/sai/sai-read-write-paths.adoc[SAI write path and read path].

== What on-disk index formats does SAI support?

SAI supports two on-disk index formats, optimized for:

* Equality and non-exact matching on *strings*.
** Strings are indexed on-disk using the https://en.wikipedia.org/wiki/Trie[trie] data structure, in conjunction with postings (term/row pairs) lists.
The trie is heap friendly, providing string prefix compression for terms, and can match any query that can be expressed as a deterministic finite automaton.
The feature minimizes on-disk footprint and supports simple token skipping.

* Equality and range queries on *numeric and non-literal types*.
** Numeric values and the other non-literal CQL types (`timestamp`, `date`, `UUID`) are indexed on-disk using https://en.wikipedia.org/wiki/K-d_tree[k-dimensional tree], a balanced structure that provides fast lookups across one or more dimensions, and compression for both values and postings.

== What is the disk footprint overhead for SAI indexes?

SAI requires significantly lower disk usage compared to other native or bolt-on Cassandra index solutions.
SAI produces an additional 20-35% disk usage compared with *unindexed* data.
The SAI disk usage is largely dependent on the underlying data model and the number of columns indexed.

== What are the supported column data types for SAI indexing?

The supported types are: `ASCII, BIGINT, DATE, DECIMAL, DOUBLE, FLOAT, INET, INT, SMALLINT, TEXT, TIME, TIMESTAMP, TIMEUUID, TINYINT, UUID, VARCHAR, VARINT`.

[NOTE]
====
* `INET` support for IPv4 and IPv6 was new starting with Cassandra ???.
* The `DECIMAL` and `VARINT` support was new starting with Cassandra ???.
* SAI also supports collections -- see the xref:#saiCollectionColumnFaq[next FAQ].
====

[[saiCollectionColumnFaq]]
== Does SAI support indexes on a collection column?

Yes -- SAI supports collections of type `map`, `list`, and `set`.
See the following topics:

* xref:cassandra:developing/cql/indexing/sai/collections.adoc[Using collections with SAI]
// * xref:reference:cql-commands/create-custom-index.adoc[CREATE CUSTOM INDEX] 

include::cassandra:partial$sai/collections-note.adoc[]

== What are the supported query operators?

For queries on tables with SAI indexes:

include::cassandra:partial$sai/supported-query-operators-list.adoc[]

The unsupported query operators are:

include::cassandra:partial$sai/notSupportedOperators.adoc[]

Examples:

[source,language-cql]
----
include::cassandra:example$CQL/sai/cyclist_semi_pro_select_date_range.cql[tag=cyclistSelectDateRange]
----

[source,language-cql]
----
SELECT * FROM audit WHERE text_map CONTAINS KEY 'Giovani';
----

[TIP]
====
For query examples with `CONTAINS` clauses that take advantage of SAI collection maps, lists, and sets, be sure to see xref:developing/cql/indexing/sai/collections.adoc[SAI collection map examples with keys, values, and entries].
====

== On the `CREATE INDEX` command for SAI, what options are available?

Use the `WITH OPTIONS` clause to indicate how SAI should handle case sensitivity and special characters in the index.
For example, given a string column `lastname`:

[source,language-cql]
----
CREATE INDEX lastname_sai_idx ON cycling.cyclist_semi_pro (lastname)
  USING 'sai' WITH OPTIONS =
  {'case_sensitive': 'false', 'normalize': 'true', 'ascii': 'true'};
----

[TIP]
====
SAI has an `ascii` option.
The default is `false`.
When set to `true`, SAI converts alphabetic, numeric, and symbolic characters that are not in the Basic Latin Unicode block (the first 127 ASCII characters) to the ASCII equivalent, if one exists.
For example, this option changes `à` to `a`.

See xref:reference/cql-commands/create-custom-index.adoc[CREATE CUSTOM INDEX] and examples in the xref:cassandra:getting-started/sai-quickstart.adoc[SAI quickstart] topic.
====

== Does SAI support composite indexing: meaning, a single index on multiple columns?

No.
There is a 1-to-1 mapping of an SAI index to a column.
However, you can create a separate index on each column in a given table.
Also, SAI can use multiple defined indexes within a single read query.

== How can I view SAI memory usage metrics?

The SAI memory footprint is divided between the JVM heap and the Chunk Cache.
The heap stores memtable indexes, and the chunk cache stores recently accessed on-disk index components as well as other SSTable components.
SAI provides metrics for both the heap and the chunk cache.
For each index, SAI also provides metrics for determining the size in bytes of memory used by the on-disk data structure, as well as disk usage.
Refer to xref:developing/cql/indexing/sai/operations/monitoring.adoc#saiIndexGroupMetrics[Index group metrics].
SAI also provides Table state metrics that give you visibility into the disk usage, the percentage of disk usage of the base table, the index builds in progress, and related metrics.
See xref:developing/cql/indexing/sai/operations/monitoring.adoc#saiTableStateMetrics[Table state metrics].


[[saiAndQueriesFaq]]
== What is the performance impact of adding SAI columns to a read query? How many `AND` clauses can I add?

There is no limit on the number of index columns that can be used in a single query.
The `sai_indexes_per_table_failure_threshold` setting in cassandra.yaml controls the maximum number of SAI indexes allowed in a single table (10, by default).
However, querying against multiple indexed columns incurs a cost that is related to the increased number of index components processed.
When evaluating multiple indexed columns in a query, SAI performs a workflow (1: Traverse.
2: Merged.
3: Intersect that ultimately coalesces data from multiple memtables and SSTables.

[NOTE]
====
In a query, AND queries will process up to two SAI indexes;
if more than two SAI indexes are used by the query, this circumstance will result in SAI performing post-filtering on the remaining clauses.
====

For related information, see the xref:cassandra:developing/cql/indexing/sai/sai-read-write-paths.adoc#saiMatchStreamingAndPostFiltering[match streaming and post filtering] example.

[[saiWritesSyncFaq]]
== Are SAI write operations asynchronous, or does SAI wait before acknowledging the write to the user?

The SAI write path is actually very simple.
The indexes live with the data, both in memtables and SSTables.
When a write is acknowledged to the client, the data is already indexed.
This is a *synchronous* process.
When the memtable is flushed, the indexes are flushed as well.
See xref:developing/cql/indexing/sai/sai-read-write-paths.adoc[SAI write path and read path].

The on-disk index components are broken down into per-SSTable index files and per-column index files.
The column indexes do not store the primary keys or tokens;
instead, they store compressible row IDs.
The per-SSTable index files link the row IDs from the column-indexes to their backing SSTables.
This SAI design allows all column indexes within a single SSTable to share per-SSTable index files, which further helps reduce the disk footprint.

== What are the guidelines regarding column cardinality with SAI indexes?

Column https://en.wikipedia.org/wiki/Cardinality_(SQL_statements)[cardinality] can affect read performance when it comes to range queries among replicas.
The number of rows matching a value of a high-cardinality column, such as credit card numbers, is more likely to be isolated on very few nodes (or even isolated to one node), while the rows matching a value on a low-cardinality column are more likely to reside on numerous nodes.
If a query does not specify a partition key, the Cassandra coordinator scans the token ring and group token ranges by endpoints (nodes).
The coordinator then concurrently execute read commands for all participating endpoints.
In the worst case where the indexed column has very high cardinality, an entire cluster scan may be required before finding a match.
With low-cardinality columns, be aware that if your `LIMIT` is higher than the number of values in your targeted column, Cassandra has to search all replicas again before determining that the `LIMIT` cannot be satisfied.
In this case, Cassandra returns only the number of matching results.

== What are the circumstances under which SAI applies post filtering?
SAI applies post-filtering in numerous scenarios.
For example, consider a simple table, and an SAI index on just one of the two non-PK columns:

[source,language-cql]
----
CREATE KEYSPACE test WITH REPLICATION = {'class': 'SimpleStrategy', 'replication_factor': 1};
CREATE TABLE test.mytable (id int PRIMARY KEY,
       col1 text,
       col2 timestamp);
CREATE CUSTOM INDEX mytable_col1_idx ON test.mytable (col1) USING 'StorageAttachedIndex';
----

Given a query such as the following:

[source,language-cql]
----
SELECT * FROM test.mytable WHERE col1 = 'hello world' and col2 < toTimestamp(now()) ALLOW FILTERING;
----

For this query, Cassandra narrows down the search by the indexed column (`col1`) first, then applies post-filtering on `col2`.
(Use the `ALLOW FILTERING` clause with caution.) In this scenario, no additional replica roundtrips are needed;
the post filtering on `col2` is carried out on the replicas themselves.

Another case were post-filtering comes into play is when constructing a query that involves more than two SAI indexes.
Refer to this xref:#saiAndQueriesFaq[related FAQ] about `AND` queries.

== Can I create an SAI index based on a xref:reference:static.adoc[static column]?

Yes.
For example, consider a `transaction_by_customer` table where you have a primary key `customer_id`, plus static columns to contain each customer's `address`, `phone_number`, and `date_of_birth`.
Given a query like:

[source,language-cql]
----
SELECT * from transaction_by_customer where customer_id = 'xyz123';
----

If there are 100,000 `transaction_by_customer` rows, because you defined those three static fields, this query runs against a table that uses significantly less disk space, as compared to an environment where writes had inserted the per-customer values (`address`, `phone_number`, `date_of_birth`) in every row.

[NOTE]
====
SAI delivers the option and advantage of creating indexes based on static columns, while also achieving the benefit of conserving table space.
====

== For indexed strings, how does SAI handle Unicode characters in the column data?

When you create an SAI index based on a string column, set the `normalize` option to `true` if you want SAI to perform Unicode normalization on the column data.
SAI supports Normalization Form C (NFC) Unicode.
When set to `true`, SAI normalizes the different versions of a given Unicode character to a single version, retaining all the marks and symbols in the index.
For example, SAI would change the character Å (U+212B) to Å (U+00C5).
See xref:reference/cql-commands/create-custom-index.adoc[CREATE CUSTOM INDEX].

== Can the index's column name have special characters?

SAI validates the column name on which an index is being defined.
SAI allows alphanumeric characters and underscores only.
SAI returns `InvalidRequestException` if you try to define an index on a column name that contains other characters, and does not create the index.

== What partitioner does SAI support?

SAI supports only the `Murmur3Partitioner`.