cspann: do not retry search when deleting vector

Previously, if the search for a vector to delete fails, the index
would fetch a second batch of partitions to search. However, if a
vector cannot be found in the first batch, it's very unlikely to be
found at all, so better to abort at that point. This is important
for the case when a split or merge is in progress and the vector is
found in a draining partition and skipped. It's a waste to keep
searching in that case.

Epic: CRDB-42943
Release note: None

Author: andy-kimball

pkg/sql/vecindex/cspann/index.go

@@ -270,7 +270,9 @@ func NewIndex(
 		vi.options.MaxInsertAttempts = 32
 	}
 	if vi.options.MaxDeleteAttempts == 0 {
-		vi.options.MaxDeleteAttempts = 3
+		// Note that fetching the batch and getting the first result from the
+		// batch are both counted as "attempts", so this needs to be at least 2.
+		vi.options.MaxDeleteAttempts = 2
 	}
 
 	if vi.options.MaxPartitionSize < 2 {

pkg/sql/vecindex/cspann/testdata/delete.ddt

@@ -94,8 +94,7 @@ vec7: (2, 8)
     │
     └───• vec4 (-4, 5)
 
-# Test case where initial search fails to find vector to delete and it must be
-# retried.
+# Test case where search fails to find vector to delete.
 delete
 vec1: (2, 4)
 ----
@@ -116,6 +115,7 @@ vec1: (2, 4)
     │
     ├───• 4 (1, -2)
     │   │
+    │   ├───• vec1 (MISSING)
     │   └───• vec6 (1, -6)
     │
     └───• 2 (5.5, 3.5)
@@ -141,6 +141,9 @@ vec6
 └───• 9 (3.25, 0.75)
     │
     ├───• 4 (1, -2)
+    │   │
+    │   └───• vec1 (MISSING)
+    │
     └───• 2 (5.5, 3.5)
         │
         ├───• vec3 (4, 3)

pkg/sql/vecindex/cspann/testdata/search-for-delete.ddt

@@ -1,7 +1,7 @@
 # ----------------------------------------------------------------------
 # Search tree with multiple partitions and duplicate data.
 # ----------------------------------------------------------------------
-new-index dims=2 min-partition-size=1 max-partition-size=4 beam-size=1
+new-index dims=2 min-partition-size=1 max-partition-size=4 beam-size=2
 vec1: (1, 2)
 vec2: (7, 4)
 vec3: (4, 3)
@@ -68,9 +68,9 @@ vec100: (1, 2)
 ----
 vec100: vector not found
 
-# Search for vector with wrong value, which forces internal retry.
+# Search for vector with wrong value that cannot be found with beam_size=2.
 search-for-delete
-vec1:(0, 9)
+vec1:(6, 2)
 ----
 vec1: partition 7