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| <section id="query-optimizer"> |
| <h1>Query Optimizer<a class="headerlink" href="#query-optimizer" title="Link to this heading">#</a></h1> |
| <p><a class="reference external" href="https://crates.io/crates/datafusion">DataFusion</a> is an extensible query execution framework, written in Rust, that uses Apache Arrow as its in-memory |
| format.</p> |
| <p>DataFusion has modular design, allowing individual crates to be re-used in other projects.</p> |
| <p>This crate is a submodule of DataFusion that provides a query optimizer for logical plans, and |
| contains an extensive set of <a class="reference external" href="https://docs.rs/datafusion/latest/datafusion/optimizer/trait.OptimizerRule.html"><code class="docutils literal notranslate"><span class="pre">OptimizerRule</span></code></a>s and <a class="reference external" href="https://docs.rs/datafusion/latest/datafusion/physical_optimizer/trait.PhysicalOptimizerRule.html"><code class="docutils literal notranslate"><span class="pre">PhysicalOptimizerRule</span></code></a>s that may rewrite the plan and/or its expressions so |
| they execute more quickly while still computing the same result.</p> |
| <p>For a deeper background on optimizer architecture and rule types and predicates, see |
| <a class="reference external" href="https://datafusion.apache.org/blog/2025/06/15/optimizing-sql-dataframes-part-one">Optimizing SQL (and DataFrames) in DataFusion, Part 1</a>, <a class="reference external" href="https://datafusion.apache.org/blog/2025/06/15/optimizing-sql-dataframes-part-two">Part 2</a>, |
| <a class="reference external" href="https://datafusion.apache.org/blog/2025/03/11/ordering-analysis">Using Ordering for Better Plans in Apache DataFusion</a>, and |
| <a class="reference external" href="https://datafusion.apache.org/blog/2025/09/10/dynamic-filters">Dynamic Filters: Passing Information Between Operators During Execution for 25x Faster Queries</a>.</p> |
| <section id="running-the-optimizer"> |
| <h2>Running the Optimizer<a class="headerlink" href="#running-the-optimizer" title="Link to this heading">#</a></h2> |
| <p>The following code demonstrates the basic flow of creating the optimizer with a default set of optimization rules |
| and applying it to a logical plan to produce an optimized logical plan.</p> |
| <div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="k">use</span><span class="w"> </span><span class="n">std</span><span class="p">::</span><span class="n">sync</span><span class="p">::</span><span class="n">Arc</span><span class="p">;</span> |
| <span class="k">use</span><span class="w"> </span><span class="n">datafusion</span><span class="p">::</span><span class="n">logical_expr</span><span class="p">::{</span><span class="n">col</span><span class="p">,</span><span class="w"> </span><span class="n">lit</span><span class="p">,</span><span class="w"> </span><span class="n">LogicalPlan</span><span class="p">,</span><span class="w"> </span><span class="n">LogicalPlanBuilder</span><span class="p">};</span> |
| <span class="k">use</span><span class="w"> </span><span class="n">datafusion</span><span class="p">::</span><span class="n">optimizer</span><span class="p">::{</span><span class="n">OptimizerRule</span><span class="p">,</span><span class="w"> </span><span class="n">OptimizerContext</span><span class="p">,</span><span class="w"> </span><span class="n">Optimizer</span><span class="p">};</span> |
| |
| <span class="c1">// We need a logical plan as the starting point. There are many ways to build a logical plan:</span> |
| <span class="c1">//</span> |
| <span class="c1">// The `datafusion-expr` crate provides a LogicalPlanBuilder</span> |
| <span class="c1">// The `datafusion-sql` crate provides a SQL query planner that can create a LogicalPlan from SQL</span> |
| <span class="c1">// The `datafusion` crate provides a DataFrame API that can create a LogicalPlan</span> |
| |
| <span class="kd">let</span><span class="w"> </span><span class="n">initial_logical_plan</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">LogicalPlanBuilder</span><span class="p">::</span><span class="n">empty</span><span class="p">(</span><span class="kc">false</span><span class="p">).</span><span class="n">build</span><span class="p">().</span><span class="n">unwrap</span><span class="p">();</span> |
| |
| <span class="c1">// use builtin rules or customized rules</span> |
| <span class="kd">let</span><span class="w"> </span><span class="n">rules</span><span class="p">:</span><span class="w"> </span><span class="nb">Vec</span><span class="o"><</span><span class="n">Arc</span><span class="o"><</span><span class="k">dyn</span><span class="w"> </span><span class="n">OptimizerRule</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="nb">Send</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="nb">Sync</span><span class="o">>></span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="fm">vec!</span><span class="p">[];</span> |
| |
| <span class="kd">let</span><span class="w"> </span><span class="n">optimizer</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Optimizer</span><span class="p">::</span><span class="n">with_rules</span><span class="p">(</span><span class="n">rules</span><span class="p">);</span> |
| |
| <span class="kd">let</span><span class="w"> </span><span class="n">config</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">OptimizerContext</span><span class="p">::</span><span class="n">new</span><span class="p">().</span><span class="n">with_max_passes</span><span class="p">(</span><span class="mi">16</span><span class="p">);</span> |
| |
| <span class="kd">let</span><span class="w"> </span><span class="n">optimized_plan</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">optimizer</span><span class="p">.</span><span class="n">optimize</span><span class="p">(</span><span class="n">initial_logical_plan</span><span class="p">.</span><span class="n">clone</span><span class="p">(),</span><span class="w"> </span><span class="o">&</span><span class="n">config</span><span class="p">,</span><span class="w"> </span><span class="n">observer</span><span class="p">);</span> |
| |
| <span class="k">fn</span><span class="w"> </span><span class="nf">observer</span><span class="p">(</span><span class="n">plan</span><span class="p">:</span><span class="w"> </span><span class="kp">&</span><span class="nc">LogicalPlan</span><span class="p">,</span><span class="w"> </span><span class="n">rule</span><span class="p">:</span><span class="w"> </span><span class="kp">&</span><span class="nc">dyn</span><span class="w"> </span><span class="n">OptimizerRule</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="fm">println!</span><span class="p">(</span> |
| <span class="w"> </span><span class="s">"After applying rule '{}':</span><span class="se">\n</span><span class="s">{}"</span><span class="p">,</span> |
| <span class="w"> </span><span class="n">rule</span><span class="p">.</span><span class="n">name</span><span class="p">(),</span> |
| <span class="w"> </span><span class="n">plan</span><span class="p">.</span><span class="n">display_indent</span><span class="p">()</span> |
| <span class="w"> </span><span class="p">)</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="writing-optimization-rules"> |
| <h2>Writing Optimization Rules<a class="headerlink" href="#writing-optimization-rules" title="Link to this heading">#</a></h2> |
| <p>Please refer to the |
| <a class="reference external" href="https://github.com/apache/datafusion/blob/main/datafusion-examples/examples/query_planning/optimizer_rule.rs">optimizer_rule.rs</a> |
| example to learn more about the general approach to writing optimizer rules and |
| then move onto studying the existing rules.</p> |
| <p><code class="docutils literal notranslate"><span class="pre">OptimizerRule</span></code> transforms one <a class="reference external" href="https://docs.rs/datafusion/latest/datafusion/logical_expr/enum.LogicalPlan.html"><code class="docutils literal notranslate"><span class="pre">LogicalPlan</span></code></a> into another which |
| computes the same results, but in a potentially more efficient |
| way. If there are no suitable transformations for the input plan, |
| the optimizer can simply return it as is.</p> |
| <p>All rules must implement the <code class="docutils literal notranslate"><span class="pre">OptimizerRule</span></code> trait.</p> |
| <div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="cp">#[derive(Default, Debug)]</span> |
| <span class="k">struct</span><span class="w"> </span><span class="nc">MyOptimizerRule</span><span class="w"> </span><span class="p">{}</span> |
| |
| <span class="k">impl</span><span class="w"> </span><span class="n">OptimizerRule</span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="n">MyOptimizerRule</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="nf">name</span><span class="p">(</span><span class="o">&</span><span class="bp">self</span><span class="p">)</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="kp">&</span><span class="kt">str</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="s">"my_optimizer_rule"</span> |
| <span class="w"> </span><span class="p">}</span> |
| |
| <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="nf">rewrite</span><span class="p">(</span> |
| <span class="w"> </span><span class="o">&</span><span class="bp">self</span><span class="p">,</span> |
| <span class="w"> </span><span class="n">plan</span><span class="p">:</span><span class="w"> </span><span class="nc">LogicalPlan</span><span class="p">,</span> |
| <span class="w"> </span><span class="n">_config</span><span class="p">:</span><span class="w"> </span><span class="kp">&</span><span class="nc">dyn</span><span class="w"> </span><span class="n">OptimizerConfig</span><span class="p">,</span> |
| <span class="w"> </span><span class="p">)</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="nb">Result</span><span class="o"><</span><span class="n">Transformed</span><span class="o"><</span><span class="n">LogicalPlan</span><span class="o">>></span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="fm">unimplemented!</span><span class="p">()</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="providing-custom-rules"> |
| <h2>Providing Custom Rules<a class="headerlink" href="#providing-custom-rules" title="Link to this heading">#</a></h2> |
| <p>The optimizer can be created with a custom set of rules.</p> |
| <div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="kd">let</span><span class="w"> </span><span class="n">optimizer</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Optimizer</span><span class="p">::</span><span class="n">with_rules</span><span class="p">(</span><span class="fm">vec!</span><span class="p">[</span> |
| <span class="w"> </span><span class="n">Arc</span><span class="p">::</span><span class="n">new</span><span class="p">(</span><span class="n">MyOptimizerRule</span><span class="w"> </span><span class="p">{})</span> |
| <span class="p">]);</span> |
| </pre></div> |
| </div> |
| <section id="general-guidelines"> |
| <h3>General Guidelines<a class="headerlink" href="#general-guidelines" title="Link to this heading">#</a></h3> |
| <p>Rules typical walk the logical plan and walk the expression trees inside operators and selectively mutate |
| individual operators or expressions.</p> |
| <p>Sometimes there is an initial pass that visits the plan and builds state that is used in a second pass that performs |
| the actual optimization. This approach is used in projection push down and filter push down.</p> |
| </section> |
| <section id="expression-naming"> |
| <h3>Expression Naming<a class="headerlink" href="#expression-naming" title="Link to this heading">#</a></h3> |
| <p>Every expression in DataFusion has a name, which is used as the column name. For example, in this example the output |
| contains a single column with the name <code class="docutils literal notranslate"><span class="pre">"COUNT(aggregate_test_100.c9)"</span></code>:</p> |
| <div class="highlight-text notranslate"><div class="highlight"><pre><span></span>> select count(c9) from aggregate_test_100; |
| +------------------------------+ |
| | COUNT(aggregate_test_100.c9) | |
| +------------------------------+ |
| | 100 | |
| +------------------------------+ |
| </pre></div> |
| </div> |
| <p>These names are used to refer to the columns in both subqueries as well as internally from one stage of the LogicalPlan |
| to another. For example:</p> |
| <div class="highlight-text notranslate"><div class="highlight"><pre><span></span>> select "COUNT(aggregate_test_100.c9)" + 1 from (select count(c9) from aggregate_test_100) as sq; |
| +--------------------------------------------+ |
| | sq.COUNT(aggregate_test_100.c9) + Int64(1) | |
| +--------------------------------------------+ |
| | 101 | |
| +--------------------------------------------+ |
| </pre></div> |
| </div> |
| </section> |
| <section id="implication"> |
| <h3>Implication<a class="headerlink" href="#implication" title="Link to this heading">#</a></h3> |
| <p>Because DataFusion identifies columns using a string name, it means it is critical that the names of expressions are |
| not changed by the optimizer when it rewrites expressions. This is typically accomplished by renaming a rewritten |
| expression by adding an alias.</p> |
| <p>Here is a simple example of such a rewrite. The expression <code class="docutils literal notranslate"><span class="pre">1</span> <span class="pre">+</span> <span class="pre">2</span></code> can be internally simplified to 3 but must still be |
| displayed the same as <code class="docutils literal notranslate"><span class="pre">1</span> <span class="pre">+</span> <span class="pre">2</span></code>:</p> |
| <div class="highlight-text notranslate"><div class="highlight"><pre><span></span>> select 1 + 2; |
| +---------------------+ |
| | Int64(1) + Int64(2) | |
| +---------------------+ |
| | 3 | |
| +---------------------+ |
| </pre></div> |
| </div> |
| <p>Looking at the <code class="docutils literal notranslate"><span class="pre">EXPLAIN</span></code> output we can see that the optimizer has effectively rewritten <code class="docutils literal notranslate"><span class="pre">1</span> <span class="pre">+</span> <span class="pre">2</span></code> into effectively |
| <code class="docutils literal notranslate"><span class="pre">3</span> <span class="pre">as</span> <span class="pre">"1</span> <span class="pre">+</span> <span class="pre">2"</span></code>:</p> |
| <div class="highlight-text notranslate"><div class="highlight"><pre><span></span>> explain format indent select 1 + 2; |
| +---------------+-------------------------------------------------+ |
| | plan_type | plan | |
| +---------------+-------------------------------------------------+ |
| | logical_plan | Projection: Int64(3) AS Int64(1) + Int64(2) | |
| | | EmptyRelation | |
| | physical_plan | ProjectionExec: expr=[3 as Int64(1) + Int64(2)] | |
| | | PlaceholderRowExec | |
| | | | |
| +---------------+-------------------------------------------------+ |
| </pre></div> |
| </div> |
| <p>If the expression name is not preserved, bugs such as <a class="reference external" href="https://github.com/apache/datafusion/issues/3704">#3704</a> |
| and <a class="reference external" href="https://github.com/apache/datafusion/issues/3555">#3555</a> occur where the expected columns can not be found.</p> |
| </section> |
| <section id="building-expression-names"> |
| <h3>Building Expression Names<a class="headerlink" href="#building-expression-names" title="Link to this heading">#</a></h3> |
| <p>There are currently two ways to create a name for an expression in the logical plan.</p> |
| <div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="k">impl</span><span class="w"> </span><span class="n">Expr</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="sd">/// Returns the name of this expression as it should appear in a schema. This name</span> |
| <span class="w"> </span><span class="sd">/// will not include any CAST expressions.</span> |
| <span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="nf">display_name</span><span class="p">(</span><span class="o">&</span><span class="bp">self</span><span class="p">)</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="nb">Result</span><span class="o"><</span><span class="nb">String</span><span class="o">></span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="nb">Ok</span><span class="p">(</span><span class="s">"display_name"</span><span class="p">.</span><span class="n">to_string</span><span class="p">())</span> |
| <span class="w"> </span><span class="p">}</span> |
| |
| <span class="w"> </span><span class="sd">/// Returns a full and complete string representation of this expression.</span> |
| <span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="nf">canonical_name</span><span class="p">(</span><span class="o">&</span><span class="bp">self</span><span class="p">)</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="nb">String</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="s">"canonical_name"</span><span class="p">.</span><span class="n">to_string</span><span class="p">()</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <p>When comparing expressions to determine if they are equivalent, <code class="docutils literal notranslate"><span class="pre">canonical_name</span></code> should be used, and when creating a |
| name to be used in a schema, <code class="docutils literal notranslate"><span class="pre">display_name</span></code> should be used.</p> |
| </section> |
| <section id="utilities"> |
| <h3>Utilities<a class="headerlink" href="#utilities" title="Link to this heading">#</a></h3> |
| <p>There are a number of <a class="reference external" href="https://github.com/apache/datafusion/blob/main/datafusion/expr/src/utils.rs">utility methods</a> provided that take care of some common tasks.</p> |
| </section> |
| <section id="recursively-walk-an-expression-tree"> |
| <h3>Recursively walk an expression tree<a class="headerlink" href="#recursively-walk-an-expression-tree" title="Link to this heading">#</a></h3> |
| <p>The <a class="reference external" href="https://docs.rs/datafusion/latest/datafusion/common/tree_node/trait.TreeNode.html">TreeNode API</a> provides a convenient way to recursively walk an expression or plan tree.</p> |
| <p>For example, to find all subquery references in a logical plan, the following code can be used:</p> |
| <div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// Return all subquery references in an expression</span> |
| <span class="k">fn</span><span class="w"> </span><span class="nf">extract_subquery_filters</span><span class="p">(</span><span class="n">expression</span><span class="p">:</span><span class="w"> </span><span class="kp">&</span><span class="nc">Expr</span><span class="p">)</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="nb">Result</span><span class="o"><</span><span class="nb">Vec</span><span class="o"><&</span><span class="n">Expr</span><span class="o">>></span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">extracted</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="fm">vec!</span><span class="p">[];</span> |
| <span class="w"> </span><span class="n">expression</span><span class="p">.</span><span class="n">apply</span><span class="p">(</span><span class="o">|</span><span class="n">expr</span><span class="o">|</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">Expr</span><span class="p">::</span><span class="n">InSubquery</span><span class="p">(</span><span class="n">_</span><span class="p">)</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">expr</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">extracted</span><span class="p">.</span><span class="n">push</span><span class="p">(</span><span class="n">expr</span><span class="p">);</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="w"> </span><span class="nb">Ok</span><span class="p">(</span><span class="n">TreeNodeRecursion</span><span class="p">::</span><span class="n">Continue</span><span class="p">)</span> |
| <span class="w"> </span><span class="p">})</span><span class="o">?</span><span class="p">;</span> |
| <span class="w"> </span><span class="nb">Ok</span><span class="p">(</span><span class="n">extracted</span><span class="p">)</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <p>Likewise you can use the <a class="reference external" href="https://docs.rs/datafusion/latest/datafusion/common/tree_node/trait.TreeNode.html">TreeNode API</a> to rewrite a <code class="docutils literal notranslate"><span class="pre">LogicalPlan</span></code> or <code class="docutils literal notranslate"><span class="pre">ExecutionPlan</span></code></p> |
| <div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// Return all joins in a logical plan</span> |
| <span class="k">fn</span><span class="w"> </span><span class="nf">find_joins</span><span class="p">(</span><span class="n">overall_plan</span><span class="p">:</span><span class="w"> </span><span class="kp">&</span><span class="nc">LogicalPlan</span><span class="p">)</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="nb">Result</span><span class="o"><</span><span class="nb">Vec</span><span class="o"><&</span><span class="n">Join</span><span class="o">>></span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">extracted</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="fm">vec!</span><span class="p">[];</span> |
| <span class="w"> </span><span class="n">overall_plan</span><span class="p">.</span><span class="n">apply</span><span class="p">(</span><span class="o">|</span><span class="n">plan</span><span class="o">|</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">LogicalPlan</span><span class="p">::</span><span class="n">Join</span><span class="p">(</span><span class="n">join</span><span class="p">)</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">plan</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">extracted</span><span class="p">.</span><span class="n">push</span><span class="p">(</span><span class="n">join</span><span class="p">);</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="w"> </span><span class="nb">Ok</span><span class="p">(</span><span class="n">TreeNodeRecursion</span><span class="p">::</span><span class="n">Continue</span><span class="p">)</span> |
| <span class="w"> </span><span class="p">})</span><span class="o">?</span><span class="p">;</span> |
| <span class="w"> </span><span class="nb">Ok</span><span class="p">(</span><span class="n">extracted</span><span class="p">)</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="rewriting-expressions"> |
| <h3>Rewriting expressions<a class="headerlink" href="#rewriting-expressions" title="Link to this heading">#</a></h3> |
| <p>The <a class="reference external" href="https://docs.rs/datafusion/latest/datafusion/common/tree_node/trait.TreeNode.html">TreeNode API</a> also provides a convenient way to rewrite expressions and |
| plans as well. For example to rewrite all expressions like</p> |
| <div class="highlight-sql notranslate"><div class="highlight"><pre><span></span><span class="n">col</span><span class="w"> </span><span class="k">BETWEEN</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="k">AND</span><span class="w"> </span><span class="n">y</span> |
| </pre></div> |
| </div> |
| <p>into</p> |
| <div class="highlight-sql notranslate"><div class="highlight"><pre><span></span><span class="n">col</span><span class="w"> </span><span class="o">>=</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="k">AND</span><span class="w"> </span><span class="n">col</span><span class="w"> </span><span class="o"><=</span><span class="w"> </span><span class="n">y</span> |
| </pre></div> |
| </div> |
| <p>you can use the following code:</p> |
| <div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// Recursively rewrite all BETWEEN expressions</span> |
| <span class="c1">// returns Transformed::yes if any changes were made</span> |
| <span class="k">fn</span><span class="w"> </span><span class="nf">rewrite_between</span><span class="p">(</span><span class="n">expr</span><span class="p">:</span><span class="w"> </span><span class="nc">Expr</span><span class="p">)</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="nb">Result</span><span class="o"><</span><span class="n">Transformed</span><span class="o"><</span><span class="n">Expr</span><span class="o">>></span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="c1">// transform_up does a bottom up rewrite</span> |
| <span class="w"> </span><span class="n">expr</span><span class="p">.</span><span class="n">transform_up</span><span class="p">(</span><span class="o">|</span><span class="n">expr</span><span class="o">|</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="c1">// only handle BETWEEN expressions</span> |
| <span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">Expr</span><span class="p">::</span><span class="n">Between</span><span class="p">(</span><span class="n">Between</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">negated</span><span class="p">,</span> |
| <span class="w"> </span><span class="n">expr</span><span class="p">,</span> |
| <span class="w"> </span><span class="n">low</span><span class="p">,</span> |
| <span class="w"> </span><span class="n">high</span><span class="p">,</span> |
| <span class="w"> </span><span class="p">})</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">expr</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="nb">Ok</span><span class="p">(</span><span class="n">Transformed</span><span class="p">::</span><span class="n">no</span><span class="p">(</span><span class="n">expr</span><span class="p">))</span> |
| <span class="w"> </span><span class="p">};</span> |
| <span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">rewritten_expr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="n">negated</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="c1">// don't rewrite NOT BETWEEN</span> |
| <span class="w"> </span><span class="n">Expr</span><span class="p">::</span><span class="n">Between</span><span class="p">(</span><span class="n">Between</span><span class="p">::</span><span class="n">new</span><span class="p">(</span><span class="n">expr</span><span class="p">,</span><span class="w"> </span><span class="n">negated</span><span class="p">,</span><span class="w"> </span><span class="n">low</span><span class="p">,</span><span class="w"> </span><span class="n">high</span><span class="p">))</span> |
| <span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="c1">// rewrite to (expr >= low) AND (expr <= high)</span> |
| <span class="w"> </span><span class="n">expr</span><span class="p">.</span><span class="n">clone</span><span class="p">().</span><span class="n">gt_eq</span><span class="p">(</span><span class="o">*</span><span class="n">low</span><span class="p">).</span><span class="n">and</span><span class="p">(</span><span class="n">expr</span><span class="p">.</span><span class="n">lt_eq</span><span class="p">(</span><span class="o">*</span><span class="n">high</span><span class="p">))</span> |
| <span class="w"> </span><span class="p">};</span> |
| <span class="w"> </span><span class="nb">Ok</span><span class="p">(</span><span class="n">Transformed</span><span class="p">::</span><span class="n">yes</span><span class="p">(</span><span class="n">rewritten_expr</span><span class="p">))</span> |
| <span class="w"> </span><span class="p">})</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="writing-tests"> |
| <h3>Writing Tests<a class="headerlink" href="#writing-tests" title="Link to this heading">#</a></h3> |
| <p>There should be unit tests in the same file as the new rule that test the effect of the rule being applied to a plan |
| in isolation (without any other rule being applied).</p> |
| <p>There should also be a test in <code class="docutils literal notranslate"><span class="pre">integration-tests.rs</span></code> that tests the rule as part of the overall optimization process.</p> |
| </section> |
| <section id="debugging"> |
| <h3>Debugging<a class="headerlink" href="#debugging" title="Link to this heading">#</a></h3> |
| <p>The <code class="docutils literal notranslate"><span class="pre">EXPLAIN</span> <span class="pre">VERBOSE</span></code> command can be used to show the effect of each optimization rule on a query.</p> |
| <p>In the following example, the <code class="docutils literal notranslate"><span class="pre">type_coercion</span></code> and <code class="docutils literal notranslate"><span class="pre">simplify_expressions</span></code> passes have simplified the plan so that it returns the constant <code class="docutils literal notranslate"><span class="pre">"3.2"</span></code> rather than doing a computation at execution time.</p> |
| <div class="highlight-text notranslate"><div class="highlight"><pre><span></span>> explain verbose select cast(1 + 2.2 as string) as foo; |
| +------------------------------------------------------------+---------------------------------------------------------------------------+ |
| | plan_type | plan | |
| +------------------------------------------------------------+---------------------------------------------------------------------------+ |
| | initial_logical_plan | Projection: CAST(Int64(1) + Float64(2.2) AS Utf8) AS foo | |
| | | EmptyRelation | |
| | logical_plan after type_coercion | Projection: CAST(CAST(Int64(1) AS Float64) + Float64(2.2) AS Utf8) AS foo | |
| | | EmptyRelation | |
| | logical_plan after simplify_expressions | Projection: Utf8("3.2") AS foo | |
| | | EmptyRelation | |
| | logical_plan after unwrap_cast_in_comparison | SAME TEXT AS ABOVE | |
| | logical_plan after decorrelate_where_exists | SAME TEXT AS ABOVE | |
| | logical_plan after decorrelate_where_in | SAME TEXT AS ABOVE | |
| | logical_plan after scalar_subquery_to_join | SAME TEXT AS ABOVE | |
| | logical_plan after subquery_filter_to_join | SAME TEXT AS ABOVE | |
| | logical_plan after simplify_expressions | SAME TEXT AS ABOVE | |
| | logical_plan after eliminate_filter | SAME TEXT AS ABOVE | |
| | logical_plan after reduce_cross_join | SAME TEXT AS ABOVE | |
| | logical_plan after common_sub_expression_eliminate | SAME TEXT AS ABOVE | |
| | logical_plan after eliminate_limit | SAME TEXT AS ABOVE | |
| | logical_plan after projection_push_down | SAME TEXT AS ABOVE | |
| | logical_plan after rewrite_disjunctive_predicate | SAME TEXT AS ABOVE | |
| | logical_plan after reduce_outer_join | SAME TEXT AS ABOVE | |
| | logical_plan after filter_push_down | SAME TEXT AS ABOVE | |
| | logical_plan after limit_push_down | SAME TEXT AS ABOVE | |
| | logical_plan after single_distinct_aggregation_to_group_by | SAME TEXT AS ABOVE | |
| | logical_plan | Projection: Utf8("3.2") AS foo | |
| | | EmptyRelation | |
| | initial_physical_plan | ProjectionExec: expr=[3.2 as foo] | |
| | | PlaceholderRowExec | |
| | | | |
| | physical_plan after aggregate_statistics | SAME TEXT AS ABOVE | |
| | physical_plan after join_selection | SAME TEXT AS ABOVE | |
| | physical_plan after coalesce_batches | SAME TEXT AS ABOVE | |
| | physical_plan after repartition | SAME TEXT AS ABOVE | |
| | physical_plan after add_merge_exec | SAME TEXT AS ABOVE | |
| | physical_plan | ProjectionExec: expr=[3.2 as foo] | |
| | | PlaceholderRowExec | |
| | | | |
| +------------------------------------------------------------+---------------------------------------------------------------------------+ |
| </pre></div> |
| </div> |
| </section> |
| </section> |
| <section id="thinking-about-query-optimization"> |
| <h2>Thinking about Query Optimization<a class="headerlink" href="#thinking-about-query-optimization" title="Link to this heading">#</a></h2> |
| <p>Query optimization in DataFusion uses a cost based model. The cost based model |
| relies on table and column level statistics to estimate selectivity; selectivity |
| estimates are an important piece in cost analysis for filters and projections |
| as they allow estimating the cost of joins and filters.</p> |
| <p>An important piece of building these estimates is <em>boundary analysis</em> which uses |
| interval arithmetic to take an expression such as <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">></span> <span class="pre">2500</span> <span class="pre">AND</span> <span class="pre">a</span> <span class="pre"><=</span> <span class="pre">5000</span></code> and |
| build an accurate selectivity estimate that can then be used to find more efficient |
| plans.</p> |
| <section id="analysiscontext-api"> |
| <h3><code class="docutils literal notranslate"><span class="pre">AnalysisContext</span></code> API<a class="headerlink" href="#analysiscontext-api" title="Link to this heading">#</a></h3> |
| <p>The <code class="docutils literal notranslate"><span class="pre">AnalysisContext</span></code> serves as a shared knowledge base during expression evaluation |
| and boundary analysis. Think of it as a dynamic repository that maintains information about:</p> |
| <ol class="arabic simple"> |
| <li><p>Current known boundaries for columns and expressions</p></li> |
| <li><p>Statistics that have been gathered or inferred</p></li> |
| <li><p>A mutable state that can be updated as analysis progresses</p></li> |
| </ol> |
| <p>What makes <code class="docutils literal notranslate"><span class="pre">AnalysisContext</span></code> particularly powerful is its ability to propagate information |
| through the expression tree. As each node in the expression tree is analyzed, it can both |
| read from and write to this shared context, allowing for sophisticated boundary analysis and inference.</p> |
| </section> |
| <section id="columnstatistics-for-cardinality-estimation"> |
| <h3><code class="docutils literal notranslate"><span class="pre">ColumnStatistics</span></code> for Cardinality Estimation<a class="headerlink" href="#columnstatistics-for-cardinality-estimation" title="Link to this heading">#</a></h3> |
| <p>Column statistics form the foundation of optimization decisions. Rather than just tracking |
| simple metrics, DataFusion’s <code class="docutils literal notranslate"><span class="pre">ColumnStatistics</span></code> provides a rich set of information including:</p> |
| <ul class="simple"> |
| <li><p>Null value counts</p></li> |
| <li><p>Maximum and minimum values</p></li> |
| <li><p>Value sums (for numeric columns)</p></li> |
| <li><p>Distinct value counts</p></li> |
| </ul> |
| <p>Each of these statistics is wrapped in a <code class="docutils literal notranslate"><span class="pre">Precision</span></code> type that indicates whether the value is |
| exact or estimated, allowing the optimizer to make informed decisions about the reliability |
| of its cardinality estimates.</p> |
| </section> |
| <section id="boundary-analysis-flow"> |
| <h3>Boundary Analysis Flow<a class="headerlink" href="#boundary-analysis-flow" title="Link to this heading">#</a></h3> |
| <p>The boundary analysis process flows through several stages, with each stage building |
| upon the information gathered in previous stages. The <code class="docutils literal notranslate"><span class="pre">AnalysisContext</span></code> is continuously |
| updated as the analysis progresses through the expression tree.</p> |
| <section id="expression-boundary-analysis"> |
| <h4>Expression Boundary Analysis<a class="headerlink" href="#expression-boundary-analysis" title="Link to this heading">#</a></h4> |
| <p>When analyzing expressions, DataFusion runs boundary analysis using interval arithmetic. |
| Consider a simple predicate like age > 18 AND age <= 25. The analysis flows as follows:</p> |
| <ol class="arabic simple"> |
| <li><p>Context Initialization</p> |
| <ul class="simple"> |
| <li><p>Begin with known column statistics</p></li> |
| <li><p>Set up initial boundaries based on column constraints</p></li> |
| <li><p>Initialize the shared analysis context</p></li> |
| </ul> |
| </li> |
| <li><p>Expression Tree Walk</p> |
| <ul class="simple"> |
| <li><p>Analyze each node in the expression tree</p></li> |
| <li><p>Propagate boundary information upward</p></li> |
| <li><p>Allow child nodes to influence parent boundaries</p></li> |
| </ul> |
| </li> |
| <li><p>Boundary Updates</p> |
| <ul class="simple"> |
| <li><p>Each expression can update the shared context</p></li> |
| <li><p>Changes flow through the entire expression tree</p></li> |
| <li><p>Final boundaries inform optimization decisions</p></li> |
| </ul> |
| </li> |
| </ol> |
| </section> |
| </section> |
| <section id="working-with-the-analysis-api"> |
| <h3>Working with the analysis API<a class="headerlink" href="#working-with-the-analysis-api" title="Link to this heading">#</a></h3> |
| <p>The following example shows how you can run an analysis pass on a physical expression |
| to infer the selectivity of the expression and the space of possible values it can |
| take.</p> |
| <div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="k">fn</span><span class="w"> </span><span class="nf">analyze_filter_example</span><span class="p">()</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="nb">Result</span><span class="o"><</span><span class="p">()</span><span class="o">></span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="c1">// Create a schema with an 'age' column</span> |
| <span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">age</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Field</span><span class="p">::</span><span class="n">new</span><span class="p">(</span><span class="s">"age"</span><span class="p">,</span><span class="w"> </span><span class="n">DataType</span><span class="p">::</span><span class="n">Int64</span><span class="p">,</span><span class="w"> </span><span class="kc">false</span><span class="p">);</span> |
| <span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">schema</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Arc</span><span class="p">::</span><span class="n">new</span><span class="p">(</span><span class="n">Schema</span><span class="p">::</span><span class="n">new</span><span class="p">(</span><span class="fm">vec!</span><span class="p">[</span><span class="n">age</span><span class="p">]));</span> |
| |
| <span class="w"> </span><span class="c1">// Define column statistics</span> |
| <span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">column_stats</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">ColumnStatistics</span><span class="p">::</span><span class="n">default</span><span class="p">()</span> |
| <span class="w"> </span><span class="p">.</span><span class="n">with_min_value</span><span class="p">(</span><span class="n">Precision</span><span class="p">::</span><span class="n">Exact</span><span class="p">(</span><span class="n">ScalarValue</span><span class="p">::</span><span class="n">Int64</span><span class="p">(</span><span class="nb">Some</span><span class="p">(</span><span class="mi">14</span><span class="p">))))</span> |
| <span class="w"> </span><span class="p">.</span><span class="n">with_max_value</span><span class="p">(</span><span class="n">Precision</span><span class="p">::</span><span class="n">Exact</span><span class="p">(</span><span class="n">ScalarValue</span><span class="p">::</span><span class="n">Int64</span><span class="p">(</span><span class="nb">Some</span><span class="p">(</span><span class="mi">79</span><span class="p">))))</span> |
| <span class="w"> </span><span class="p">.</span><span class="n">with_null_count</span><span class="p">(</span><span class="n">Precision</span><span class="p">::</span><span class="n">Exact</span><span class="p">(</span><span class="mi">0</span><span class="p">));</span> |
| |
| <span class="w"> </span><span class="c1">// Create expression: age > 18 AND age <= 25</span> |
| <span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">expr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">col</span><span class="p">(</span><span class="s">"age"</span><span class="p">)</span> |
| <span class="w"> </span><span class="p">.</span><span class="n">gt</span><span class="p">(</span><span class="n">lit</span><span class="p">(</span><span class="mi">18</span><span class="k">i64</span><span class="p">))</span> |
| <span class="w"> </span><span class="p">.</span><span class="n">and</span><span class="p">(</span><span class="n">col</span><span class="p">(</span><span class="s">"age"</span><span class="p">).</span><span class="n">lt_eq</span><span class="p">(</span><span class="n">lit</span><span class="p">(</span><span class="mi">25</span><span class="k">i64</span><span class="p">)));</span> |
| |
| <span class="w"> </span><span class="c1">// Initialize analysis context</span> |
| <span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">initial_boundaries</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="fm">vec!</span><span class="p">[</span><span class="n">ExprBoundaries</span><span class="p">::</span><span class="n">try_from_column</span><span class="p">(</span> |
| <span class="w"> </span><span class="o">&</span><span class="n">schema</span><span class="p">,</span><span class="w"> </span><span class="o">&</span><span class="n">column_stats</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">)</span><span class="o">?</span><span class="p">];</span> |
| <span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">context</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">AnalysisContext</span><span class="p">::</span><span class="n">new</span><span class="p">(</span><span class="n">initial_boundaries</span><span class="p">);</span> |
| |
| <span class="w"> </span><span class="c1">// Analyze expression</span> |
| <span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">df_schema</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">DFSchema</span><span class="p">::</span><span class="n">try_from</span><span class="p">(</span><span class="n">schema</span><span class="p">)</span><span class="o">?</span><span class="p">;</span> |
| <span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">physical_expr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">SessionContext</span><span class="p">::</span><span class="n">new</span><span class="p">().</span><span class="n">create_physical_expr</span><span class="p">(</span><span class="n">expr</span><span class="p">,</span><span class="w"> </span><span class="o">&</span><span class="n">df_schema</span><span class="p">)</span><span class="o">?</span><span class="p">;</span> |
| <span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">analysis</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">analyze</span><span class="p">(</span><span class="o">&</span><span class="n">physical_expr</span><span class="p">,</span><span class="w"> </span><span class="n">context</span><span class="p">,</span><span class="w"> </span><span class="n">df_schema</span><span class="p">.</span><span class="n">as_ref</span><span class="p">())</span><span class="o">?</span><span class="p">;</span> |
| |
| <span class="w"> </span><span class="nb">Ok</span><span class="p">(())</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| </section> |
| </section> |
| </section> |
| |
| |
| </article> |
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