214 lines
7.6 KiB
HTML
214 lines
7.6 KiB
HTML
<!DOCTYPE html>
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<html><head>
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<meta name="viewport" content="width=device-width, initial-scale=1.0">
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<meta http-equiv="content-type" content="text/html; charset=UTF-8">
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<link href="sqlite.css" rel="stylesheet">
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<title>The UNION Virtual Table</title>
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<!-- path= -->
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</head>
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<body>
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<div class=nosearch>
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<a href="index.html">
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<div><!-- IE hack to prevent disappearing logo --></div>
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<div class="tagline desktoponly">
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Small. Fast. Reliable.<br>Choose any three.
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</div>
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<div class="menu mainmenu">
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<li><a href="index.html">Home</a>
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</div>
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<div class=fancy>
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<div class=nosearch>
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<div class="fancy_title">
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The UNION Virtual Table
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</div>
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</div>
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<ol>
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<li value="100"><p>
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The UNION virtual table (hereafter: "union-vtab")
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is a <a href="vtab.html">virtual table</a> that makes multiple independent
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<a href="rowidtable.html">rowid tables</a> tables look like a single large table.
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</p></li><li value="110"><p>
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The tables that participate in a union-vtab can be in the same
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database file, or they can be in separate databases files that
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are <a href="lang_attach.html">ATTACH</a>-ed to the same database connection.
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</p></li><li value="120"><p>
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The union-vtab is not built into SQLite.
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Union-vtab is a <a href="loadext.html">loadable extension</a>.
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The source code for union-vtab is contained in a single file located at
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<a href="https://sqlite.org/src/file/ext/misc/unionvtab.c">ext/misc/unionvtab.c</a>
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in the SQLite source tree.
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</p></li><li value="200"><p>
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A new union-vtab instance is created as follows:
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</p><blockquote>
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<b>CREATE VIRTUAL TABLE temp.</b><i>tabname</i> <b>USING unionvtab(</b><i>query</i><b>);</b>
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</blockquote>
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</li><li value="210"><p>
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Every union-vtab must be in the TEMP namespace. Hence, the "<b>temp.</b>"
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prior to <i>tabname</i> is required. Only the union-vtab itself is required
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to be in the TEMP namespace - the individual tables that are being unioned
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can be any <a href="lang_attach.html">ATTACH</a>-ed database.
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</p></li><li value="220"><p>
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The <i>query</i> in the CREATE VIRTUAL TABLE statement for a union-vtab
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must be a well-formed SQL query that returns four columns and an
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arbitrary number of rows. Each row in the result of the <i>query</i>
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represents a single table that is to participate in the union.
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</p><ol>
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<li value="221'">
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The first column is the schema name for the database that contains
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the tables. Examples: "main", "zone512".
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</li><li value="222'">
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The second column is the name of the table.
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</li><li value="223'">
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The third column is the minimum value for any rowid in the table.
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</li><li value="224'">
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The fourth column is the maximum value of any rowid in the table.
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</li></ol>
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</li><li value="230"><p>
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The <i>query</i> for the CREATE VIRTUAL TABLE statement of a union-vtab
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can be either a <a href="lang_select.html">SELECT</a> statement or a <a href="lang_select.html#values">VALUES clause</a>.
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</p></li><li value="240"><p>
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The <i>query</i> is run once when the CREATE VIRTUAL TABLE statement is
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first encountered and the results of that one run are used for all subsequent
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access to the union-vtab. If the results of <i>query</i> change, then
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the union-vtab should be <a href="lang_droptable.html">DROP</a>-ed and recreated in order
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to cause the <i>query</i> to be run again.
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</p></li><li value="250"><p>
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There must be no overlap in the bands of rowids for the various tables
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in a union-vtab.
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</p></li><li value="260"><p>
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All tables that participate in a union-vtab must have identical
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CREATE TABLE definitions, except that the names of the tables can be different.
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</p></li><li value="270"><p>
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All tables that participate in a union-vtab must be <a href="rowidtable.html">rowid tables</a>.
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</p></li><li value="280"><p>
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The column names and definitions for <i>tabname</i> will be the same as
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the underlying tables. An application can access <i>tabname</i> just like
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it was one of the real underlying tables.
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</p></li><li value="290"><p>
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No table in a union-vtab may contain entries that are outside of the
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rowid bounds established by the <i>query</i> in the CREATE VIRTUAL TABLE
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statement.
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</p></li><li value="300"><p>
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The union-vtab shall optimize access to the underlying real tables
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when the constraints on the query are among forms shown below.
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Other kinds of constraints may be optimized in the future, but only
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these constraints are optimized in the initial implementation.
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</p><ul>
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<li> <b>rowid=$id</b>
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</li><li> <b>rowid IN</b> <i>query-or-list</i>
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</li><li> <b>rowid BETWEEN $lwr AND $upr</b>
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</li></ul>
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<p>
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Other kinds of constraints may be used and will work, but other
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constraints will be checked individually for each row and will not
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be optimized (at least not initially).
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All constraint checking is completely automatic regardless of whether
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or not optimization occurs. The optimization referred to in this bullet point
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is a performance consideration only. The same result is obtained
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regardless of whether or not the query is optimized.
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</p></li><li value="310"><p>
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The union-vtab is read-only. Support for writing may be added at a later
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time, but writing is not a part of the initial implementation.
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</p></li><li value="320"><p>
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<i>Nota bene:</i>
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The <a href="c3ref/blob_open.html">sqlite3_blob_open()</a> interface does <u>not</u> work for a union-vtab.
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BLOB content must be read from the union-vtab using ordinary SQL statements.
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</p></li></ol>
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