add approx_distinct implementation

README.md

@@ -190,6 +190,8 @@ DataFusion also includes a simple command-line interactive SQL utility. See the
   - [x] trim
 - Miscellaneous/Boolean functions
   - [x] nullif
+- Approximation functions
+  - [ ] approx_distinct
 - Common date/time functions
   - [ ] Basic date functions
   - [ ] Basic time functions

ballista/rust/core/proto/ballista.proto

@@ -167,6 +167,7 @@ enum AggregateFunction {
   SUM = 2;
   AVG = 3;
   COUNT = 4;
+  APPROX_DISTINCT = 5;
 }
 
 message AggregateExprNode {

ballista/rust/core/src/serde/logical_plan/to_proto.rs

@@ -1137,6 +1137,9 @@ impl TryInto<protobuf::LogicalExprNode> for &Expr {
                 ref fun, ref args, ..
             } => {
                 let aggr_function = match fun {
+                    AggregateFunction::ApproxDistinct => {
+                        protobuf::AggregateFunction::ApproxDistinct
+                    }
                     AggregateFunction::Min => protobuf::AggregateFunction::Min,
                     AggregateFunction::Max => protobuf::AggregateFunction::Max,
                     AggregateFunction::Sum => protobuf::AggregateFunction::Sum,
@@ -1370,6 +1373,7 @@ impl From<&AggregateFunction> for protobuf::AggregateFunction {
             AggregateFunction::Sum => Self::Sum,
             AggregateFunction::Avg => Self::Avg,
             AggregateFunction::Count => Self::Count,
+            AggregateFunction::ApproxDistinct => Self::ApproxDistinct,
         }
     }
 }

ballista/rust/core/src/serde/mod.rs

@@ -114,6 +114,9 @@ impl From<protobuf::AggregateFunction> for AggregateFunction {
             protobuf::AggregateFunction::Sum => AggregateFunction::Sum,
             protobuf::AggregateFunction::Avg => AggregateFunction::Avg,
             protobuf::AggregateFunction::Count => AggregateFunction::Count,
+            protobuf::AggregateFunction::ApproxDistinct => {
+                AggregateFunction::ApproxDistinct
+            }
         }
     }
 }

datafusion/src/logical_plan/expr.rs

@@ -1495,6 +1495,21 @@ pub fn random() -> Expr {
     }
 }
 
+/// Returns the approximate number of distinct input values.
+/// This function provides an approximation of count(DISTINCT x).
+/// Zero is returned if all input values are null.
+/// This function should produce a standard error of 2.3%,
+/// which is the standard deviation of the (approximately normal)
+/// error distribution over all possible sets.
+/// It does not guarantee an upper bound on the error for any specific input set.
+pub fn approx_distinct(expr: Expr) -> Expr {
+    Expr::AggregateFunction {
+        fun: aggregates::AggregateFunction::Sum,
+        distinct: false,
+        args: vec![expr],
+    }
+}
+
 /// Create an convenience function representing a unary scalar function
 macro_rules! unary_scalar_expr {
     ($ENUM:ident, $FUNC:ident) => {

datafusion/src/logical_plan/mod.rs

@@ -36,17 +36,17 @@ pub use builder::{
 pub use dfschema::{DFField, DFSchema, DFSchemaRef, ToDFSchema};
 pub use display::display_schema;
 pub use expr::{
-    abs, acos, and, array, ascii, asin, atan, avg, binary_expr, bit_length, btrim, case,
-    ceil, character_length, chr, col, columnize_expr, combine_filters, concat, concat_ws,
-    cos, count, count_distinct, create_udaf, create_udf, date_part, date_trunc, digest,
-    exp, exprlist_to_fields, floor, in_list, initcap, left, length, lit,
-    lit_timestamp_nano, ln, log10, log2, lower, lpad, ltrim, max, md5, min,
-    normalize_col, normalize_cols, now, octet_length, or, random, regexp_match,
-    regexp_replace, repeat, replace, replace_col, reverse, right, round, rpad, rtrim,
-    sha224, sha256, sha384, sha512, signum, sin, split_part, sqrt, starts_with, strpos,
-    substr, sum, tan, to_hex, translate, trim, trunc, unnormalize_col, unnormalize_cols,
-    upper, when, Column, Expr, ExprRewriter, ExpressionVisitor, Literal, Recursion,
-    RewriteRecursion,
+    abs, acos, and, approx_distinct, array, ascii, asin, atan, avg, binary_expr,
+    bit_length, btrim, case, ceil, character_length, chr, col, columnize_expr,
+    combine_filters, concat, concat_ws, cos, count, count_distinct, create_udaf,
+    create_udf, date_part, date_trunc, digest, exp, exprlist_to_fields, floor, in_list,
+    initcap, left, length, lit, lit_timestamp_nano, ln, log10, log2, lower, lpad, ltrim,
+    max, md5, min, normalize_col, normalize_cols, now, octet_length, or, random,
+    regexp_match, regexp_replace, repeat, replace, replace_col, reverse, right, round,
+    rpad, rtrim, sha224, sha256, sha384, sha512, signum, sin, split_part, sqrt,
+    starts_with, strpos, substr, sum, tan, to_hex, translate, trim, trunc,
+    unnormalize_col, unnormalize_cols, upper, when, Column, Expr, ExprRewriter,
+    ExpressionVisitor, Literal, Recursion, RewriteRecursion,
 };
 pub use extension::UserDefinedLogicalNode;
 pub use operators::Operator;

datafusion/src/physical_plan/aggregates.rs

@@ -59,6 +59,8 @@ pub enum AggregateFunction {
     Max,
     /// avg
     Avg,
+    /// Approximate aggregate function
+    ApproxDistinct,
 }
 
 impl fmt::Display for AggregateFunction {
@@ -77,6 +79,7 @@ impl FromStr for AggregateFunction {
             "count" => AggregateFunction::Count,
             "avg" => AggregateFunction::Avg,
             "sum" => AggregateFunction::Sum,
+            "approx_distinct" => AggregateFunction::ApproxDistinct,
             _ => {
                 return Err(DataFusionError::Plan(format!(
                     "There is no built-in function named {}",
@@ -96,7 +99,9 @@ pub fn return_type(fun: &AggregateFunction, arg_types: &[DataType]) -> Result<Da
     data_types(arg_types, &signature(fun))?;
 
     match fun {
-        AggregateFunction::Count => Ok(DataType::UInt64),
+        AggregateFunction::Count | AggregateFunction::ApproxDistinct => {
+            Ok(DataType::UInt64)
+        }
         AggregateFunction::Max | AggregateFunction::Min => Ok(arg_types[0].clone()),
         AggregateFunction::Sum => sum_return_type(&arg_types[0]),
         AggregateFunction::Avg => avg_return_type(&arg_types[0]),
@@ -149,6 +154,9 @@ pub fn create_aggregate_expr(
                 "SUM(DISTINCT) aggregations are not available".to_string(),
             ));
         }
+        (AggregateFunction::ApproxDistinct, _) => Arc::new(
+            expressions::ApproxDistinct::new(arg, name, arg_types[0].clone()),
+        ),
         (AggregateFunction::Min, _) => {
             Arc::new(expressions::Min::new(arg, name, return_type))
         }
@@ -194,7 +202,9 @@ static DATES: &[DataType] = &[DataType::Date32, DataType::Date64];
 pub fn signature(fun: &AggregateFunction) -> Signature {
     // note: the physical expression must accept the type returned by this function or the execution panics.
     match fun {
-        AggregateFunction::Count => Signature::any(1, Volatility::Immutable),
+        AggregateFunction::Count | AggregateFunction::ApproxDistinct => {
+            Signature::any(1, Volatility::Immutable)
+        }
         AggregateFunction::Min | AggregateFunction::Max => {
             let valid = STRINGS
                 .iter()