feat(cost-model): Add filter implementation (#41)

* Add filter implementation

* modify the comment

Author: lanlou1554

optd-cost-model/src/cost/filter.rs

@@ -0,0 +1,182 @@
+use std::ops::Bound;
+
+use crate::{
+    common::{types::TableId, values::Value},
+    cost_model::CostModelImpl,
+    stats::{AttributeCombValue, AttributeCombValueStats, DEFAULT_EQ_SEL, DEFAULT_INEQ_SEL},
+    storage::CostModelStorageManager,
+    CostModelResult,
+};
+
+impl<S: CostModelStorageManager> CostModelImpl<S> {
+    /// Get the selectivity of an expression of the form "attribute equals value" (or "value equals
+    /// attribute") Will handle the case of statistics missing
+    /// Equality predicates are handled entirely differently from range predicates so this is its
+    /// own function
+    /// Also, get_attribute_equality_selectivity is a subroutine when computing range selectivity,
+    /// which is another reason for separating these into two functions is_eq means whether it's == or !=
+    ///
+    /// Currently, we only support calculating the equality selectivity for an existed attribute,
+    /// not a derived attribute.
+    /// TODO: Support derived attributes.
+    pub(crate) async fn get_attribute_equality_selectivity(
+        &self,
+        table_id: TableId,
+        attr_base_index: u64,
+        value: &Value,
+        is_eq: bool,
+    ) -> CostModelResult<f64> {
+        let ret_sel = {
+            if let Some(attribute_stats) = self
+                .get_attribute_comb_stats(table_id, &[attr_base_index])
+                .await?
+            {
+                let eq_freq =
+                    if let Some(freq) = attribute_stats.mcvs.freq(&vec![Some(value.clone())]) {
+                        freq
+                    } else {
+                        let non_mcv_freq = 1.0 - attribute_stats.mcvs.total_freq();
+                        // always safe because usize is at least as large as i32
+                        let ndistinct_as_usize = attribute_stats.ndistinct as usize;
+                        let non_mcv_cnt = ndistinct_as_usize - attribute_stats.mcvs.cnt();
+                        if non_mcv_cnt == 0 {
+                            return Ok(0.0);
+                        }
+                        // note that nulls are not included in ndistinct so we don't need to do non_mcv_cnt
+                        // - 1 if null_frac > 0
+                        (non_mcv_freq - attribute_stats.null_frac) / (non_mcv_cnt as f64)
+                    };
+                if is_eq {
+                    eq_freq
+                } else {
+                    1.0 - eq_freq - attribute_stats.null_frac
+                }
+            } else {
+                #[allow(clippy::collapsible_else_if)]
+                if is_eq {
+                    DEFAULT_EQ_SEL
+                } else {
+                    1.0 - DEFAULT_EQ_SEL
+                }
+            }
+        };
+
+        assert!(
+            (0.0..=1.0).contains(&ret_sel),
+            "ret_sel ({}) should be in [0, 1]",
+            ret_sel
+        );
+        Ok(ret_sel)
+    }
+
+    /// Compute the frequency of values in a attribute less than or equal to the given value.
+    fn get_attribute_leq_value_freq(
+        per_attribute_stats: &AttributeCombValueStats,
+        value: &Value,
+    ) -> f64 {
+        // because distr does not include the values in MCVs, we need to compute the CDFs there as
+        // well because nulls return false in any comparison, they are never included when
+        // computing range selectivity
+        let distr_leq_freq = per_attribute_stats.distr.as_ref().unwrap().cdf(value);
+        let value = value.clone();
+        let pred = Box::new(move |val: &AttributeCombValue| *val[0].as_ref().unwrap() <= value);
+        let mcvs_leq_freq = per_attribute_stats.mcvs.freq_over_pred(pred);
+        let ret_freq = distr_leq_freq + mcvs_leq_freq;
+        assert!(
+            (0.0..=1.0).contains(&ret_freq),
+            "ret_freq ({}) should be in [0, 1]",
+            ret_freq
+        );
+        ret_freq
+    }
+
+    /// Compute the frequency of values in a attribute less than the given value.
+    ///
+    /// Currently, we only support calculating the equality selectivity for an existed attribute,
+    /// not a derived attribute.
+    /// TODO: Support derived attributes.
+    async fn get_attribute_lt_value_freq(
+        &self,
+        attribute_stats: &AttributeCombValueStats,
+        table_id: TableId,
+        attr_base_index: u64,
+        value: &Value,
+    ) -> CostModelResult<f64> {
+        // depending on whether value is in mcvs or not, we use different logic to turn total_lt_cdf
+        // into total_leq_cdf this logic just so happens to be the exact same logic as
+        // get_attribute_equality_selectivity implements
+        let ret_freq = Self::get_attribute_leq_value_freq(attribute_stats, value)
+            - self
+                .get_attribute_equality_selectivity(table_id, attr_base_index, value, true)
+                .await?;
+        assert!(
+            (0.0..=1.0).contains(&ret_freq),
+            "ret_freq ({}) should be in [0, 1]",
+            ret_freq
+        );
+        Ok(ret_freq)
+    }
+
+    /// Get the selectivity of an expression of the form "attribute </<=/>=/> value" (or "value
+    /// </<=/>=/> attribute"). Computes selectivity based off of statistics.
+    /// Range predicates are handled entirely differently from equality predicates so this is its
+    /// own function. If it is unable to find the statistics, it returns DEFAULT_INEQ_SEL.
+    /// The selectivity is computed as quantile of the right bound minus quantile of the left bound.
+    ///
+    /// Currently, we only support calculating the equality selectivity for an existed attribute,
+    /// not a derived attribute.
+    /// TODO: Support derived attributes.
+    pub(crate) async fn get_attribute_range_selectivity(
+        &self,
+        table_id: TableId,
+        attr_base_index: u64,
+        start: Bound<&Value>,
+        end: Bound<&Value>,
+    ) -> CostModelResult<f64> {
+        // TODO: Consider attribute is a derived attribute
+        if let Some(attribute_stats) = self
+            .get_attribute_comb_stats(table_id, &[attr_base_index])
+            .await?
+        {
+            let left_quantile = match start {
+                Bound::Unbounded => 0.0,
+                Bound::Included(value) => {
+                    self.get_attribute_lt_value_freq(
+                        &attribute_stats,
+                        table_id,
+                        attr_base_index,
+                        value,
+                    )
+                    .await?
+                }
+                Bound::Excluded(value) => {
+                    Self::get_attribute_leq_value_freq(&attribute_stats, value)
+                }
+            };
+            let right_quantile = match end {
+                Bound::Unbounded => 1.0,
+                Bound::Included(value) => {
+                    Self::get_attribute_leq_value_freq(&attribute_stats, value)
+                }
+                Bound::Excluded(value) => {
+                    self.get_attribute_lt_value_freq(
+                        &attribute_stats,
+                        table_id,
+                        attr_base_index,
+                        value,
+                    )
+                    .await?
+                }
+            };
+            assert!(
+                left_quantile <= right_quantile,
+                "left_quantile ({}) should be <= right_quantile ({})",
+                left_quantile,
+                right_quantile
+            );
+            Ok(right_quantile - left_quantile)
+        } else {
+            Ok(DEFAULT_INEQ_SEL)
+        }
+    }
+}