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+Indexes On Expressions
+</div>
+</div>
+
+
+
+
+
+<p>
+Normally, an SQL index references columns of a table.  But an index
+can also be formed on expressions involving table columns.
+
+</p><p>
+As an example, consider the following table that tracks
+dollar-amount changes on various "accounts":
+
+</p><div class="codeblock"><pre>CREATE TABLE account_change(
+  chng_id INTEGER PRIMARY KEY,
+  acct_no INTEGER REFERENCES account,
+  location INTEGER REFERENCES locations,
+  amt INTEGER,  -- in cents
+  authority TEXT,
+  comment TEXT
+);
+CREATE INDEX acctchng_magnitude ON account_change(acct_no, abs(amt));
+</pre></div>
+
+<p>
+Each entry in the account_change table records a deposit or a withdrawal
+into an account.  Deposits have a positive "amt" and withdrawals have
+a negative "amt".
+
+</p><p>
+The acctchng_magnitude index is over the account number ("acct_no") and
+on the absolute value of the amount.  This index allows one to do 
+efficient queries over the magnitude of a change to the account.
+For example, to list all changes to account number $xyz that are
+more than $100.00, one can say:
+
+
+</p><div class="codeblock"><pre>SELECT * FROM account_change WHERE acct_no=$xyz AND abs(amt)>=10000;
+</pre></div>
+
+<p>
+Or, to list all changes to one particular account ($xyz) in order of
+decreasing magnitude, one can write:
+
+</p><div class="codeblock"><pre>SELECT * FROM account_change WHERE acct_no=$xyz
+ ORDER BY abs(amt) DESC;
+</pre></div>
+
+<p>
+Both of the above example queries would work fine without the
+acctchng_magnitude index.
+The acctchng_magnitude index merely helps the queries to run
+faster, especially on databases where there are many entries in
+the table for each account.
+
+</p><h1 id="how_to_use_indexes_on_expressions"><span>1. </span>How To Use Indexes On Expressions</h1>
+
+<p>
+Use a <a href="lang_createindex.html">CREATE INDEX</a> statement to create a new index on one or more
+expressions just like you would to create an index on columns.  The only
+difference is that expressions are listed as the elements to be indexed
+rather than column names.
+
+</p><p>
+The SQLite query planner will consider using an index on an expression
+when the expression that is indexed appears in the WHERE clause or in
+the ORDER BY clause of a query, <i>exactly</i> as it is written in the
+CREATE INDEX statement.  The query planner does not do algebra.  In order
+to match WHERE clause constraints and ORDER BY terms to indexes, SQLite
+requires that the expressions be the same, except for minor syntactic
+differences such as white-space changes.  So if you have:
+
+</p><div class="codeblock"><pre>CREATE TABLE t2(x,y,z);
+CREATE INDEX t2xy ON t2(x+y);
+</pre></div>
+
+<p>
+And then you run the query:
+
+</p><div class="codeblock"><pre>SELECT * FROM t2 WHERE y+x=22;
+</pre></div>
+
+<p>
+Then the index will <u>not</u> be used because 
+the expression on the CREATE INDEX
+statement (x+y) is not the same as the expression as it appears in the 
+query (y+x).  The two expressions might be mathematically equivalent, but
+the SQLite query planner insists that they be the same, not merely
+equivalent.  Consider rewriting the query thusly:
+
+</p><div class="codeblock"><pre>SELECT * FROM t2 WHERE x+y=22;
+</pre></div>
+
+<p>
+This second query will likely use the index because now the expression
+in the WHERE clause (x+y) matches the expression in the index exactly.
+
+
+</p><h1 id="restrictions"><span>2. </span>Restrictions</h1>
+
+<p>
+There are certain reasonable restrictions on expressions that appear in
+CREATE INDEX statements:
+
+</p><ol>
+<li><p>
+Expressions in CREATE INDEX statements
+may only refer to columns of the table being indexed, not to
+columns in other tables.
+
+</p></li><li><p>
+Expressions in CREATE INDEX statements
+may contain function calls, but only to functions whose output
+is always determined completely by its input parameters (a.k.a.:
+<a href="deterministic.html">deterministic functions</a>).  Obviously, functions like <a href="lang_corefunc.html#random">random()</a> will not
+work well in an index.  But also functions like <a href="lang_corefunc.html#sqlite_version">sqlite_version()</a>, though
+they are constant across any one database connection, are not constant
+across the life of the underlying database file, and hence may not be
+used in a CREATE INDEX statement.
+
+</p><p>
+Note that <a href="appfunc.html">application-defined SQL functions</a> are by default considered
+non-deterministic and may not be used in a CREATE INDEX statement unless
+the <a href="c3ref/c_deterministic.html#sqlitedeterministic">SQLITE_DETERMINISTIC</a> flag is used when the function is registered.
+
+</p></li><li><p>
+Expressions in CREATE INDEX statements may not use subqueries.
+
+</p></li><li><p>
+Expressions may only be used in CREATE INDEX statements, not within
+<a href="lang_createtable.html#uniqueconst">UNIQUE</a> or <a href="lang_createtable.html#primkeyconst">PRIMARY KEY</a> constraints within the <a href="lang_createtable.html">CREATE TABLE</a> statement.
+</p></li></ol>
+
+
+<h1 id="compatibility"><span>3. </span>Compatibility</h1>
+
+<p>
+The ability to index expressions was added to SQLite with 
+<a href="releaselog/3_9_0.html">version 3.9.0</a> (2015-10-14).  A database that uses an index on
+expressions will not be usable by earlier versions of SQLite.
+</p><p align="center"><small><i>This page last modified on  <a href="https://sqlite.org/docsrc/honeypot" id="mtimelink"  data-href="https://sqlite.org/docsrc/finfo/pages/expridx.in?m=918a2f1493">2023-02-11 20:57:33</a> UTC </small></i></p>
+