From a495b6c227cb72e1c88a70a984617ebb4e0e2b8b Mon Sep 17 00:00:00 2001
From: Seebs <seebs@seebs.net>
Date: Wed, 11 Mar 2020 11:03:24 -0500
Subject: [PATCH] make Distinct work across nodes, probably

Problem: A top-level bare "Distinct" call returns results only
for shards on the current node.

Analysis: We don't actually want to limit Distinct calls to "available"
shards at all. We just want to run them on everything. But we already
did that in generating the precomputed results; all we need to do is,
if we get a non-shard-specific request for precomputed values, just
return all the values.

It's pretty hard to create logic for this using our fancy mapReduce,
but also we could just... not do that.
---
 executor.go | 33 +++++----------------------------
 1 file changed, 5 insertions(+), 28 deletions(-)

diff --git a/executor.go b/executor.go
index 482a10e6e..36b1a6cda 100644
--- a/executor.go
+++ b/executor.go
@@ -1006,37 +1006,14 @@ func (e *executor) executeMaxRow(ctx context.Context, index string, c *pql.Call,
 
 // executePrecomputedCall pretends to execute a call that we have a precomputed value for.
 func (e *executor) executePrecomputedCall(ctx context.Context, index string, c *pql.Call, shards []uint64, opt *execOptions) (*Row, error) {
-	span, ctx := tracing.StartSpanFromContext(ctx, "Executor.executePrecomputedCall")
+	span, _ := tracing.StartSpanFromContext(ctx, "Executor.executePrecomputedCall")
 	defer span.Finish()
+	result := NewRow()
 
-	// Execute calls in bulk on each remote node and merge.
-	mapFn := func(shard uint64) (interface{}, error) {
-		if c.Precomputed != nil {
-			return c.Precomputed[shard], nil
-		}
-		// This might not be an error -- if there were no values, we will not have created
-		// the corresponding row.
-		return NewRow(), nil
+	for _, row := range c.Precomputed {
+		result.Merge(row.(*Row))
 	}
-
-	// Merge returned results at coordinating node.
-	reduceFn := func(prev, v interface{}) interface{} {
-		other, _ := prev.(*Row)
-		if other == nil {
-			other = NewRow()
-		}
-		other.Merge(v.(*Row))
-		return other
-	}
-
-	other, err := e.mapReduce(ctx, index, shards, c, opt, mapFn, reduceFn)
-	if err != nil {
-		return nil, errors.Wrap(err, "map reduce")
-	}
-
-	row, _ := other.(*Row)
-
-	return row, nil
+	return result, nil
 }
 
 // executeBitmapCall executes a call that returns a bitmap.