From 04171082eacf2c2c1d7c8ae868fa1dc560b619c2 Mon Sep 17 00:00:00 2001
From: namrathamk <85421654+namrathamk@users.noreply.github.com>
Date: Tue, 9 Aug 2022 04:03:14 +0530
Subject: [PATCH 1/2] Core, API: Support scanning from a tag

---
 api/src/main/java/org/apache/iceberg/TableScan.java  | 12 ++++++++++++
 .../main/java/org/apache/iceberg/DataTableScan.java  |  8 ++++++++
 2 files changed, 20 insertions(+)

diff --git a/api/src/main/java/org/apache/iceberg/TableScan.java b/api/src/main/java/org/apache/iceberg/TableScan.java
index 56f7f11d3ccd..0ab448bda497 100644
--- a/api/src/main/java/org/apache/iceberg/TableScan.java
+++ b/api/src/main/java/org/apache/iceberg/TableScan.java
@@ -49,6 +49,18 @@ public interface TableScan extends Scan<TableScan, FileScanTask, CombinedScanTas
    */
   TableScan asOfTime(long timestampMillis);
 
+  /**
+   * Create a new {@link TableScan} from this scan's configuration that will use the
+   * snapshot referenced by a tag.
+   *
+   * @param tag a snapshot ref of type tag.
+   * @return a new scan based on this with the given snapshot ref tag
+   * @throws IllegalArgumentException if the tag cannot be found
+   */
+  default TableScan useTag(String tag) {
+    throw new UnsupportedOperationException("Scanning from a tag is not supported");
+  }
+
   /**
    * Create a new {@link TableScan} from this that will read the given data columns. This produces
    * an expected schema that includes all fields that are either selected or used by this scan's
diff --git a/core/src/main/java/org/apache/iceberg/DataTableScan.java b/core/src/main/java/org/apache/iceberg/DataTableScan.java
index 2d125ae79b9d..fb2317bcb599 100644
--- a/core/src/main/java/org/apache/iceberg/DataTableScan.java
+++ b/core/src/main/java/org/apache/iceberg/DataTableScan.java
@@ -68,6 +68,14 @@ public TableScan useSnapshot(long scanSnapshotId) {
         tableOps(), table(), snapshotSchema, context().useSnapshotId(scanSnapshotId));
   }
 
+  @Override
+  public TableScan useTag(String tag) {
+    Preconditions.checkArgument(table().snapshot(tag) != null, "Cannot find tag with name %s", tag);
+    Preconditions.checkArgument(
+        table().refs().get(tag).isTag(), "Ref %s is a branch, not a tag", tag);
+    return useSnapshot(table().snapshot(tag).snapshotId());
+  }
+
   @Override
   protected TableScan newRefinedScan(
       TableOperations ops, Table table, Schema schema, TableScanContext context) {

From 093815e33ee853bef4ce068000cb0cd89504f80c Mon Sep 17 00:00:00 2001
From: namrathamk <85421654+namrathamk@users.noreply.github.com>
Date: Tue, 9 Aug 2022 18:59:15 +0530
Subject: [PATCH 2/2] Core, API: Support scanning from a tag

---
 api/src/main/java/org/apache/iceberg/TableScan.java      | 4 ++--
 core/src/main/java/org/apache/iceberg/DataTableScan.java | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/api/src/main/java/org/apache/iceberg/TableScan.java b/api/src/main/java/org/apache/iceberg/TableScan.java
index 0ab448bda497..fd0eac238511 100644
--- a/api/src/main/java/org/apache/iceberg/TableScan.java
+++ b/api/src/main/java/org/apache/iceberg/TableScan.java
@@ -50,8 +50,8 @@ public interface TableScan extends Scan<TableScan, FileScanTask, CombinedScanTas
   TableScan asOfTime(long timestampMillis);
 
   /**
-   * Create a new {@link TableScan} from this scan's configuration that will use the
-   * snapshot referenced by a tag.
+   * Create a new {@link TableScan} from this scan's configuration that will use the snapshot
+   * referenced by a tag.
    *
    * @param tag a snapshot ref of type tag.
    * @return a new scan based on this with the given snapshot ref tag
diff --git a/core/src/main/java/org/apache/iceberg/DataTableScan.java b/core/src/main/java/org/apache/iceberg/DataTableScan.java
index fb2317bcb599..a1538b93b24b 100644
--- a/core/src/main/java/org/apache/iceberg/DataTableScan.java
+++ b/core/src/main/java/org/apache/iceberg/DataTableScan.java
@@ -70,7 +70,7 @@ public TableScan useSnapshot(long scanSnapshotId) {
 
   @Override
   public TableScan useTag(String tag) {
-    Preconditions.checkArgument(table().snapshot(tag) != null, "Cannot find tag with name %s", tag);
+    Preconditions.checkArgument(tag != null, "Tag does not exist: %s", tag);
     Preconditions.checkArgument(
         table().refs().get(tag).isTag(), "Ref %s is a branch, not a tag", tag);
     return useSnapshot(table().snapshot(tag).snapshotId());