Clickhouse: allow FINAL modifier (#3534)

* allow final modifier in clickhouse dialect

* Update src/sqlfluff/dialects/dialect_clickhouse.py

Co-authored-by: Barry Pollard <barrypollard@google.com>

* Update test/fixtures/dialects/clickhouse/final.sql

Co-authored-by: Barry Pollard <barrypollard@google.com>

Co-authored-by: Barry Pollard <barrypollard@google.com>
Co-authored-by: Barry Pollard <barry_pollard@hotmail.com>

src/sqlfluff/dialects/dialect_clickhouse.py

@@ -3,15 +3,15 @@
 https://clickhouse.com/
 """
 
+from sqlfluff.core.dialects import load_raw_dialect
 from sqlfluff.core.parser import (
     Bracketed,
     Matchable,
     OneOf,
+    OptionallyBracketed,
     Ref,
     Sequence,
 )
-
-from sqlfluff.core.dialects import load_raw_dialect
 from sqlfluff.dialects import dialect_ansi as ansi
 
 ansi_dialect = load_raw_dialect("ansi")
@@ -46,3 +46,31 @@ class CTEDefinitionSegment(ansi.CTEDefinitionSegment):
             Ref("SingleIdentifierGrammar"),
         ),
     )
+
+
+class FromExpressionElementSegment(ansi.FromExpressionElementSegment):
+    """A table expression.
+
+    Overridden from ANSI to allow FINAL modifier.
+    https://clickhouse.com/docs/en/sql-reference/statements/select/from#final-modifier
+    """
+
+    type = "from_expression_element"
+    match_grammar: Matchable = Sequence(
+        Ref("PreTableFunctionKeywordsGrammar", optional=True),
+        OptionallyBracketed(Ref("TableExpressionSegment")),
+        Ref(
+            "AliasExpressionSegment",
+            exclude=OneOf(
+                Ref("SamplingExpressionSegment"),
+                Ref("JoinLikeClauseGrammar"),
+                Ref.keyword("Final"),
+            ),
+            optional=True,
+        ),
+        Ref.keyword("FINAL", optional=True),
+        # https://cloud.google.com/bigquery/docs/reference/standard-sql/arrays#flattening_arrays
+        Sequence("WITH", "OFFSET", Ref("AliasExpressionSegment"), optional=True),
+        Ref("SamplingExpressionSegment", optional=True),
+        Ref("PostTableExpressionGrammar", optional=True),
+    )

test/fixtures/dialects/clickhouse/final.sql

@@ -0,0 +1,7 @@
+select a
+from my_table final
+where a > 0;
+
+SELECT sum(bytes)
+FROM system.parts as table_alias final
+WHERE active;

test/fixtures/dialects/clickhouse/final.yml

@@ -0,0 +1,64 @@
+# YML test files are auto-generated from SQL files and should not be edited by
+# hand. To help enforce this, the "hash" field in the file must match a hash
+# computed by SQLFluff when running the tests. Please run
+# `python test/generate_parse_fixture_yml.py`  to generate them after adding or
+# altering SQL files.
+_hash: 1d3370a426ccdd899c3d108235a7fb028f8644a7b30c867df3912357ed9231b9
+file:
+- statement:
+    select_statement:
+      select_clause:
+        keyword: select
+        select_clause_element:
+          column_reference:
+            identifier: a
+      from_clause:
+        keyword: from
+        from_expression:
+          from_expression_element:
+            table_expression:
+              table_reference:
+                identifier: my_table
+            keyword: final
+      where_clause:
+        keyword: where
+        expression:
+          column_reference:
+            identifier: a
+          comparison_operator:
+            raw_comparison_operator: '>'
+          literal: '0'
+- statement_terminator: ;
+- statement:
+    select_statement:
+      select_clause:
+        keyword: SELECT
+        select_clause_element:
+          function:
+            function_name:
+              function_name_identifier: sum
+            bracketed:
+              start_bracket: (
+              expression:
+                column_reference:
+                  identifier: bytes
+              end_bracket: )
+      from_clause:
+        keyword: FROM
+        from_expression:
+          from_expression_element:
+            table_expression:
+              table_reference:
+              - identifier: system
+              - dot: .
+              - identifier: parts
+            alias_expression:
+              keyword: as
+              identifier: table_alias
+            keyword: final
+      where_clause:
+        keyword: WHERE
+        expression:
+          column_reference:
+            identifier: active
+- statement_terminator: ;