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Clustered Indexes and the WITHOUT ROWID Optimization

# 1. Introduction

By default, every row in SQLite has a special column, usually called the "rowid", that uniquely identifies that row within the table. However if the phrase "WITHOUT ROWID" is added to the end of a CREATE TABLE statement, then the special "rowid" column is omitted. There are sometimes space and performance advantages to omitting the rowid.

A WITHOUT ROWID table is a table that uses a Clustered Index as the primary key.

## 1.1. Syntax

To create a WITHOUT ROWID table, simply add the keywords "WITHOUT ROWID" to the end of the CREATE TABLE statement. For example:

>     CREATE TABLE IF NOT EXISTS wordcount(
>       word TEXT PRIMARY KEY,
>       cnt INTEGER
>     ) WITHOUT ROWID;

As with all SQL syntax, the case of the keywords does not matter. One can write "WITHOUT rowid" or "without rowid" or "WiThOuT rOwId" and it will mean the same thing.

Every WITHOUT ROWID table must have a PRIMARY KEY. An error is raised if a CREATE TABLE statement with the WITHOUT ROWID clause lacks a PRIMARY KEY.

In most contexts, the special "rowid" column of normal tables can also be called "oid" or "_rowid_". However, only "rowid" works as the keyword in the CREATE TABLE statement.

## 1.2. Compatibility

SQLite version 3.8.2 (2013-12-06) or later is necessary in order to use a WITHOUT ROWID table. An attempt to open a database that contains one or more WITHOUT ROWID tables using an earlier version of SQLite will result in a "malformed database schema" error.

## 1.3. Quirks

WITHOUT ROWID is found only in SQLite and is not compatible with any other SQL database engine, as far as we know. In an elegant system, all tables would behave as WITHOUT ROWID tables even without the WITHOUT ROWID keyword. However, when SQLite was first designed, it used only integer rowids for row keys to simplify the implementation. This approach worked well for many years. But as the demands on SQLite grew, the need for tables in which the PRIMARY KEY really did correspond to the underlying row key grew more acute. The WITHOUT ROWID concept was added in order to meet that need without breaking backwards compatibility with the billions of SQLite databases already in use at the time (circa 2013).

# 2. Differences From Ordinary Rowid Tables

The WITHOUT ROWID syntax is an optimization. It provides no new capabilities. Anything that can be done using a WITHOUT ROWID table can also be done in exactly the same way, and exactly the same syntax, using an ordinary rowid table. The only advantage of a WITHOUT ROWID table is that it can sometimes use less disk space and/or perform a little faster than an ordinary rowid table.

For the most part, ordinary rowid tables and WITHOUT ROWID tables are interchangeable. But there are some additional restrictions on WITHOUT ROWID tables that do not apply to ordinary rowid tables:

1. Every WITHOUT ROWID table must have a PRIMARY KEY. An attempt to create a WITHOUT ROWID table without a PRIMARY KEY results in an error.

2. The special behaviors associated "INTEGER PRIMARY KEY" do not apply on WITHOUT ROWID tables. In an ordinary table, "INTEGER PRIMARY KEY" means that the column is an alias for the rowid. But since there is no rowid in a WITHOUT ROWID table, that special meaning no longer applies. An "INTEGER PRIMARY KEY" column in a WITHOUT ROWID table works like an "INT PRIMARY KEY" column in an ordinary table: It is a PRIMARY KEY that has integer affinity.

3. AUTOINCREMENT does not work on WITHOUT ROWID tables. The AUTOINCREMENT mechanism assumes the presence of a rowid and so it does not work on a WITHOUT ROWID table. An error is raised if the "AUTOINCREMENT" keyword is used in the CREATE TABLE statement for a WITHOUT ROWID table.

4. NOT NULL is enforced on every column of the PRIMARY KEY in a WITHOUT ROWID table. This is in accordance with the SQL standard. Each column of a PRIMARY KEY is supposed to be individually NOT NULL. However, NOT NULL was not enforced on PRIMARY KEY columns by early versions of SQLite due to a bug. By the time that this bug was discovered, so many SQLite databases were already in circulation that the decision was made not to fix this bug for fear of breaking compatibility. So, ordinary rowid tables in SQLite violate the SQL standard and allow NULL values in PRIMARY KEY fields. But WITHOUT ROWID tables do follow the standard and will throw an error on any attempt to insert a NULL into a PRIMARY KEY column.

5. The sqlite3_last_insert_rowid() function does not work for WITHOUT ROWID tables. Inserts into a WITHOUT ROWID do not change the value returned by the sqlite3_last_insert_rowid() function. The last_insert_rowid() SQL function is also unaffected since it is just a wrapper around sqlite3_last_insert_rowid().

6. The incremental blob I/O mechanism does not work for WITHOUT ROWID tables. Incremental BLOB I/O uses the rowid to create an sqlite3_blob object for doing the direct I/O. However, WITHOUT ROWID tables do not have a rowid, and so there is no way to create an sqlite3_blob object for a WITHOUT ROWID table.

7. The sqlite3_update_hook() interface does not fire callbacks for changes to a WITHOUT ROWID table. Part of the callback from sqlite3_update_hook() is the rowid of the table row that has changed. However, WITHOUT ROWID tables do not have a rowid. Hence, the update hook is not invoked when a WITHOUT ROWID table changes.

   Note that since the session extension uses the update hook, that means that the session extension will not work correctly on a database that includes WITHOUT ROWID tables.

# 3. Benefits Of WITHOUT ROWID Tables

A WITHOUT ROWID table is an optimization that can reduce storage and processing requirements.

In an ordinary SQLite table, the PRIMARY KEY is really just a UNIQUE index. The key used to look up records on disk is the rowid. The special "INTEGER PRIMARY KEY" column type in ordinary SQLite tables causes the column to be an alias for the rowid, and so an INTEGER PRIMARY KEY is a true PRIMARY KEY. But any other kind of PRIMARY KEYs, including "INT PRIMARY KEY" are just unique indexes in an ordinary rowid table.

Consider a table (shown below) intended to store a vocabulary of words together with a count of the number of occurrences of each word in some text corpus:

>     CREATE TABLE IF NOT EXISTS wordcount(
>       word TEXT PRIMARY KEY,
>       cnt INTEGER
>     );

As an ordinary SQLite table, "wordcount" is implemented as two separate B-Trees. The main table uses the hidden rowid value as the key and stores the "word" and "cnt" columns as data. The "TEXT PRIMARY KEY" phrase of the CREATE TABLE statement causes the creation of an unique index on the "word" column. This index is a separate B-Tree that uses "word" and the "rowid" as the key and stores no data at all. Note that the complete text of every "word" is stored twice: once in the main table and again in the index.

Consider querying this table to find the number of occurrences of the word "xyzzy".:

>     SELECT cnt FROM wordcount WHERE word='xyzzy';

This query first has to search the index B-Tree looking for any entry that contains the matching value for "word". When an entry is found in the index, the rowid is extracted and used to search the main table. Then the "cnt" value is read out of the main table and returned. Hence, two separate binary searches are required to fulfill the request.

A WITHOUT ROWID table uses a different data design for the equivalent table.

>     CREATE TABLE IF NOT EXISTS wordcount(
>       word TEXT PRIMARY KEY,
>       cnt INTEGER
>     ) WITHOUT ROWID;

In this latter table, there is only a single B-Tree which uses the "word" column as its key and the "cnt" column as its data. (Technicality: the low-level implementation actually stores both "word" and "cnt" in the "key" area of the B-Tree. But unless you are looking at the low-level byte encoding of the database file, that fact is unimportant.) Because there is only a single B-Tree, the text of the "word" column is only stored once in the database. Furthermore, querying the "cnt" value for a specific "word" only involves a single binary search into the main B-Tree, since the "cnt" value can be retrieved directly from the record found by that first search and without the need to do a second binary search on the rowid.

Thus, in some cases, a WITHOUT ROWID table can use about half the amount of disk space and can operate nearly twice as fast. Of course, in a real-world schema, there will typically be secondary indices and/or UNIQUE constraints, and the situation is more complicated. But even then, there can often be space and performance advantages to using WITHOUT ROWID on tables that have non-integer or composite PRIMARY KEYs.

# 4. When To Use WITHOUT ROWID

The WITHOUT ROWID optimization is likely to be helpful for tables that have non-integer or composite (multi-column) PRIMARY KEYs and that do not store large strings or BLOBs.

WITHOUT ROWID tables will work correctly (that is to say, they provide the correct answer) for tables with a single INTEGER PRIMARY KEY. However, ordinary rowid tables will run faster in that case. Hence, it is good design to avoid creating WITHOUT ROWID tables with single-column PRIMARY KEYs of type INTEGER.

WITHOUT ROWID tables work best when individual rows are not too large. A good rule-of-thumb is that the average size of a single row in a WITHOUT ROWID table should be less than about 1/20th the size of a database page. That means that rows should not contain more than about 50 bytes each for a 1KiB page size or about 200 bytes each for 4KiB page size. WITHOUT ROWID tables will work (in the sense that they get the correct answer) for arbitrarily large rows - up to 2GB in size - but traditional rowid tables tend to work faster for large row sizes. This is because rowid tables are implemented as B*-Trees where all content is stored in the leaves of the tree, whereas WITHOUT ROWID tables are implemented using ordinary B-Trees with content stored on both leaves and intermediate nodes. Storing content in intermediate nodes mean that each intermediate node entry takes up more space on the page and thus reduces the fan-out, increasing the search cost.

The "sqlite3_analyzer.exe" utility program, available as source code in the SQLite source tree or as a precompiled binary on the SQLite Download page, can be used to measure the average sizes of table rows in an existing SQLite database.

Note that except for a few corner-case differences detailed above, WITHOUT ROWID tables and rowid tables work the same. They both generate the same answers given the same SQL statements. So it is a simple matter to run experiments on an application, late in the development cycle, to test whether or not the use of WITHOUT ROWID tables will be helpful. A good strategy is to simply not worry about WITHOUT ROWID until near the end of product development, then go back and run tests to see if adding WITHOUT ROWID to tables with non-integer PRIMARY KEYs helps or hurts performance, and retaining the WITHOUT ROWID only in those cases where it helps.