242 lines
9.3 KiB
HTML
242 lines
9.3 KiB
HTML
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<div class="fancy_title">
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The SQLITE_STMT Virtual Table
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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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The SQLITE_STMT extension implements an <a href="vtab.html#epoonlyvtab">eponymous-only virtual table</a> that
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provides information about all <a href="c3ref/stmt.html">prepared statements</a> associated with
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the <a href="c3ref/sqlite3.html">database connection</a>.
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</p>
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<p>
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The SQLITE_STMT extension is included in the <a href="amalgamation.html">amalgamation</a> though
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it is disabled
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by default. Use the <a href="compile.html#enable_stmtvtab">SQLITE_ENABLE_STMTVTAB</a> compile-time option to enable
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the SQLITE_STMT extension. The SQLITE_STMT extension can also be
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loaded at run-time
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by compiling the extension into a shared library or DLL using the source
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code at <a href="https://sqlite.org/src/file/ext/misc/stmt.c">https://sqlite.org/src/file/ext/misc/stmt.c</a> and following the
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instructions for how to <a href="loadext.html#build">compile loadable extensions</a>.
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</p>
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<p>
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The SQLITE_STMT extension is enabled in default builds
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of the <a href="cli.html">command-line shell</a>.
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</p><h1 id="usage"><span>2. </span>Usage</h1>
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<p>
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The SQLITE_STMT virtual table is a read-only table that can be directly
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queried to access information about all prepared statements on the
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current database connection. For example:
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</p><div class="codeblock"><pre>SELECT * FROM sqlite_stmt;
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</pre></div>
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<p>
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A statement such as the above can be run immediately prior to invoking
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<a href="c3ref/close.html">sqlite3_close()</a> to confirm that all prepared statements have been
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<a href="c3ref/finalize.html">finalized</a> and to help identify and track down prepared
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statements that have "leaked" and missed finalization.
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</p><p>
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The SQLITE_STMT virtual table can also be used to access performance
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information about prepared statements, to aid in optimization an application.
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For example,
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to find out how much memory is being used by <a href="c3ref/stmt.html">prepared statements</a> that have
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never been used, one could run:
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</p><div class="codeblock"><pre>SELECT sum(mem) FROM sqlite_stmt WHERE run=0;
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</pre></div>
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<h2 id="columns"><span>2.1. </span>Columns</h2>
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<p>
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The columns are provided by the SQLITE_STMT virtual table are summarized by
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the hypothetical CREATE TABLE statement show here:
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</p><div class="codeblock"><pre>CREATE TABLE sqlite_stmt(
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sql TEXT, -- Original SQL text
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ncol INT, -- Number of output columns
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ro BOOLEAN, -- True for "read only" statements
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busy BOOLEAN, -- True if the statement is current running
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nscan INT, -- Number of full-scan steps
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nsort INT, -- Number of sort operations
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naidx INT, -- Number of automatic index inserts
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nstep INT, -- Number of byte-code engine steps
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reprep INT, -- Number of reprepare operations
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run INT, -- Number of times this statement has been run
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mem INT -- Heap memory used by this statement
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);
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</pre></div>
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<p>Future releases may add new output columns and may change the order
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of legacy columns.
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Further detail about the meaning of each column in the SQLITE_STMT virtual
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table is provided below:
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</p><ul>
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<li><p><b>sql</b>:
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The original SQL text of the prepared statement. If the prepared
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statement is compiled using the <a href="c3ref/prepare.html">sqlite3_prepare()</a> interface, then
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the SQL text might not have been saved, in which case this column
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will be NULL.
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</p></li><li><p><b>ncol</b>:
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The number of columns in the result set of a query.
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For DML statements, this column has a value of 0.
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</p></li><li><p><b>ro</b>:
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The "read only" column. This column is true (non-zero) if the
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SQL statement is a query and false (zero) if it is a DML statement.
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</p></li><li><p><b>busy</b>:
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This field is true if the prepared statement is currently running.
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In other words, this field is true if <a href="c3ref/step.html">sqlite3_step()</a> has been called
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on the <a href="c3ref/stmt.html">prepared statement</a> at least once but <a href="c3ref/reset.html">sqlite3_reset()</a> has
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not yet been called to reset it.
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</p></li><li><p><b>nscan</b>:
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This field is the number of times that the <a href="opcode.html">bytecode engine</a> has stepped
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through a table as part of a full-table scan. A large number if this
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field may indicate an opportunity to improve performance by adding an
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index. This field is equivalent to the <a href="c3ref/c_stmtstatus_counter.html#sqlitestmtstatusfullscanstep">SQLITE_STMTSTATUS_FULLSCAN_STEP</a>
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value.
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</p></li><li><p><b>nsort</b>:
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This field is the number of times that the <a href="opcode.html">bytecode engine</a> had to sort.
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A positive value in this field may indicate an opportunity to improve
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performance by adding an index that will cause the query results to
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appear naturally in the desired order.
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This field is equivalent to the <a href="c3ref/c_stmtstatus_counter.html#sqlitestmtstatussort">SQLITE_STMTSTATUS_SORT</a> value.
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</p></li><li><p><b>naidx</b>:
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This field is the number of rows that have been inserted into
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<a href="optoverview.html#autoindex">automatic indexes</a>. A positive value in this field may indicate
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an opportunity to improve performance by adding a named index that
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take the place of the automatic index.
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This field is equivalent to the <a href="c3ref/c_stmtstatus_counter.html#sqlitestmtstatusautoindex">SQLITE_STMTSTATUS_AUTOINDEX</a> value.
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</p></li><li><p><b>nstep</b>:
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This field is the number of <a href="opcode.html">bytecode engine</a> operations that have
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been performed for the prepared statement. This field can be used
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as a proxy for how much CPU time a statement has used.
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This field is equivalent to the <a href="c3ref/c_stmtstatus_counter.html#sqlitestmtstatusvmstep">SQLITE_STMTSTATUS_VM_STEP</a> value.
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</p></li><li><p><b>reprep</b>:
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This field is the number of times that the statement has had to be
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reprepared due to schema changes or changes to parameter bindings.
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This field is equivalent to the <a href="c3ref/c_stmtstatus_counter.html#sqlitestmtstatusreprepare">SQLITE_STMTSTATUS_REPREPARE</a> value.
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</p></li><li><p><b>run</b>:
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This field is the number of times that the statement has been run.
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This field is equivalent to the <a href="c3ref/c_stmtstatus_counter.html#sqlitestmtstatusrun">SQLITE_STMTSTATUS_RUN</a> value.
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</p></li><li><p><b>mem</b>:
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This field is the number of bytes of heap storage used by the
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prepared statement.
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This field is equivalent to the <a href="c3ref/c_stmtstatus_counter.html#sqlitestmtstatusmemused">SQLITE_STMTSTATUS_MEMUSED</a> value.
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</p></li></ul>
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