improve unique constraint check (#337)

fixes #336

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - add exceptions for `ban-concurrent-index-creation-in-transaction` to handle golang-migrate. Thanks @janrueth! (#339)
+- improve `disallowed-unique-constraint` to handle `alter table...add column... unique`. (#337)
 
 ## v0.27.0 - 2024-01-11
 

docs/docs/disallowed-unique-constraint.md

@@ -29,6 +29,23 @@ CREATE UNIQUE INDEX CONCURRENTLY dist_id_uniq ON distributors (dist_id);
 ```
 
 
+Instead of:
+
+```sql
+-- blocks reads and writes to table_name while constraint is validated.
+ALTER TABLE distributors
+    ADD COLUMN dist_id text CONSTRAINT dist_id_uniq UNIQUE (dist_id);
+```
+
+Use:
+
+```sql
+-- add nullable column separately to prevent blocking read/writes.
+ALTER TABLE distributors ADD COLUMN dist_id text;
+-- allows reads and writes while index is built
+CREATE UNIQUE INDEX CONCURRENTLY dist_id_uniq ON distributors (dist_id);
+```
+
 ## solution for alembic and sqlalchemy
 
 ```python

linter/src/rules/disallow_unique_constraint.rs

@@ -3,7 +3,7 @@ use crate::versions::Version;
 use crate::violations::{RuleViolation, RuleViolationKind};
 
 use squawk_parser::ast::{
-    AlterTableCmds, AlterTableDef, AlterTableType, ConstrType, RawStmt, Stmt,
+    AlterTableCmds, AlterTableDef, AlterTableType, ColumnDefConstraint, ConstrType, RawStmt, Stmt,
 };
 
 #[must_use]
@@ -36,6 +36,17 @@ pub fn disallow_unique_constraint(
                                 ));
                             }
                         }
+                        (Some(AlterTableDef::ColumnDef(col)), AlterTableType::AddColumn) => {
+                            for ColumnDefConstraint::Constraint(constraint) in &col.constraints {
+                                if constraint.contype == ConstrType::Unique {
+                                    errs.push(RuleViolation::new(
+                                        RuleViolationKind::DisallowedUniqueConstraint,
+                                        raw_stmt.into(),
+                                        None,
+                                    ));
+                                }
+                            }
+                        }
                         _ => continue,
                     }
                 }
@@ -144,4 +155,18 @@ ALTER TABLE products ADD CONSTRAINT sku_constraint UNIQUE (sku);
         "#;
         assert_eq!(lint_sql_assuming_in_transaction(sql), vec![]);
     }
+    #[test]
+    fn test_unique_constraint_inline_add_column() {
+        let sql = r#"
+ALTER TABLE foo ADD COLUMN bar text CONSTRAINT foo_bar_unique UNIQUE;
+    "#;
+        assert_debug_snapshot!(lint_sql(sql));
+    }
+    #[test]
+    fn test_unique_constraint_inline_add_column_unique() {
+        let sql = r#"
+ALTER TABLE foo ADD COLUMN bar text UNIQUE;
+"#;
+        assert_debug_snapshot!(lint_sql(sql));
+    }
 }

linter/src/rules/snapshots/squawk_linter__rules__disallow_unique_constraint__test_rules__unique_constraint_inline_add_column.snap

@@ -0,0 +1,23 @@
+---
+source: linter/src/rules/disallow_unique_constraint.rs
+expression: lint_sql(sql)
+---
+[
+    RuleViolation {
+        kind: DisallowedUniqueConstraint,
+        span: Span {
+            start: 0,
+            len: Some(
+                69,
+            ),
+        },
+        messages: [
+            Note(
+                "Adding a UNIQUE constraint requires an ACCESS EXCLUSIVE lock which blocks reads.",
+            ),
+            Help(
+                "Create an index CONCURRENTLY and create the constraint using the index.",
+            ),
+        ],
+    },
+]

linter/src/rules/snapshots/squawk_linter__rules__disallow_unique_constraint__test_rules__unique_constraint_inline_add_column_unique.snap

@@ -0,0 +1,23 @@
+---
+source: linter/src/rules/disallow_unique_constraint.rs
+expression: lint_sql(sql)
+---
+[
+    RuleViolation {
+        kind: DisallowedUniqueConstraint,
+        span: Span {
+            start: 0,
+            len: Some(
+                43,
+            ),
+        },
+        messages: [
+            Note(
+                "Adding a UNIQUE constraint requires an ACCESS EXCLUSIVE lock which blocks reads.",
+            ),
+            Help(
+                "Create an index CONCURRENTLY and create the constraint using the index.",
+            ),
+        ],
+    },
+]