210 lines
8.2 KiB
HTML
210 lines
8.2 KiB
HTML
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<div class="fancy_title">
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Deterministic SQL Functions
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</div>
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</div>
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<h1 id="overview"><span>1. </span>Overview</h1>
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<p>
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SQL functions in SQLite can be either "deterministic" or "non-deterministic".
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</p><p>
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A deterministic function always gives the same answer when it has
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the same inputs. Most built-in SQL functions in SQLite are
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deterministic. For example, the <a href="lang_corefunc.html#abs">abs(X)</a> function always returns
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the same answer as long as its input X is the same.
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</p><p>
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Non-deterministic functions might give different answers on each
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invocation, even if the arguments are always the same. The following
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are examples of non-deterministic functions:
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</p><ul>
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<li> <a href="lang_corefunc.html#random">random()</a>
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</li><li> <a href="lang_corefunc.html#changes">changes()</a>
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</li><li> <a href="lang_corefunc.html#last_insert_rowid">last_insert_rowid()</a>
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</li><li> <a href="c3ref/libversion.html">sqlite3_version()</a>
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</li></ul>
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<p>
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The <a href="lang_corefunc.html#random">random()</a> function is obviously non-deterministic because it gives
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a different answer every time it is invoked. The answers from <a href="lang_corefunc.html#changes">changes()</a>
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and <a href="lang_corefunc.html#last_insert_rowid">last_insert_rowid()</a> depend on prior SQL statements, and so they
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are also non-deterministic. The
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<a href="c3ref/libversion.html">sqlite3_version()</a> function is mostly constant, but it can change when
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SQLite is upgraded, and so even though it always returns the same answer
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for any particular session, because it can change answers across sessions
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it is still considered non-deterministic.
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</p><h1 id="restrictions_on_the_use_of_non_deterministic_functions"><span>2. </span>Restrictions on the use of non-deterministic functions</h1>
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<p>
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There are some contexts in SQLite that do not allow the use of
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non-deterministic functions:
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</p><ul>
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<li>In the expression of a <a href="lang_createtable.html#ckconst">CHECK constraint</a>.
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</li><li>In the WHERE clause of a <a href="partialindex.html">partial index</a>.
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</li><li>In an expression used as part of an <a href="expridx.html">expression index</a>.
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</li><li>In the expression of a <a href="gencol.html">generated column</a>.
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</li></ul>
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<p>
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In the cases above, the values returned by the function affects the
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information stored in the database file. The values of functions
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in CHECK constraints determines which entries are valid for a table,
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and functions in the WHERE clause of a partial index or in an index on
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an expression compute values stored in the index b-tree.
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If any of these functions later returns a different
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value, then the database might no longer be well-formed.
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Hence, to avoid database corruption,
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only deterministic functions can be used in the contexts
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above.
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<a name="dtexception"></a>
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</p><h1 id="special_case_processing_for_date_time_functions"><span>3. </span>Special-case Processing For Date/Time Functions</h1>
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<p>
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The built-in <a href="lang_datefunc.html">date and time functions</a> of SQLite are a special case.
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These functions are usually considered deterministic. However, if
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these functions use the string "now" as the date, or if they use
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the <a href="lang_datefunc.html#localtime">localtime modifier</a> or the <a href="lang_datefunc.html#localtime">utc modifier</a>, then they are
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considered non-deterministic. Because the function inputs are
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not necessarily known until run-time, the date/time functions will
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throw an exception if they encounter any of the non-deterministic
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features in a context where only deterministic functions are allowed.
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</p><p>
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Prior to SQLite 3.20.0 (2017-08-01) all date/time functions were
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always considered non-deterministic. The ability for date/time functions
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to be deterministic sometimes and non-deterministic at other times,
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depending on their arguments, was added for the 3.20.0 release.
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</p><h1 id="application_defined_deterministic_functions"><span>4. </span>Application-defined deterministic functions</h1>
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<p>
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By default, <a href="appfunc.html">application-defined SQL functions</a> are considered to
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be non-deterministic. However, if the 4th parameter to
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<a href="c3ref/create_function.html">sqlite3_create_function_v2()</a> is OR-ed with
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<a href="c3ref/c_deterministic.html#sqlitedeterministic">SQLITE_DETERMINISTIC</a>, then SQLite will treat that function as if it
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were deterministic.
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</p><p>
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Note that if a non-deterministic function is tagged with
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<a href="c3ref/c_deterministic.html#sqlitedeterministic">SQLITE_DETERMINISTIC</a> and if that function ends up being used in
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the WHERE clause of a <a href="partialindex.html">partial index</a> or in an
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<a href="expridx.html">expression index</a>, then when the function begins to return different
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answers, the associated index may become corrupt. If an SQL function
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is nearly deterministic (which is to say, if it only rarely changes,
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like <a href="lang_corefunc.html#sqlite_version">sqlite_version()</a>) and it is used in an index that becomes
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corrupt, the corruption can be fixed by running <a href="lang_reindex.html">REINDEX</a>.
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</p><p>
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The interfaces necessary to construct a function that is sometimes
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deterministic and sometimes non-deterministic depending on their
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inputs, such as the built-in date/time functions, are not published.
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Generic <a href="appfunc.html">application-defined SQL functions</a> must
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be always deterministic or always non-deterministic.
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</p>
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