Add TODO.md documenting remaining parser and explain issues

Author: claude

TODO.md

@@ -0,0 +1,136 @@
+# TODO: Remaining Parser and Explain Issues
+
+## Current State
+
+- **Tests passing:** 5,197 (76.2%)
+- **Tests skipped:** 1,627 (23.8%)
+  - Parser issues: ~675
+  - Explain mismatches: ~637
+
+## Parser Issues
+
+These require changes to `parser/parser.go`:
+
+### Table/Database Names Starting with Numbers
+Tables and databases with names starting with digits fail to parse:
+```sql
+DROP TABLE IF EXISTS 03657_gby_overflow;
+DROP DATABASE IF EXISTS 03710_database;
+```
+
+### FORMAT Null
+The `FORMAT Null` clause is not recognized:
+```sql
+SELECT ... FORMAT Null;
+```
+
+### FETCH FIRST ... ROW ONLY
+SQL standard fetch syntax is not supported:
+```sql
+SELECT ... FETCH FIRST 1 ROW ONLY;
+```
+
+### INSERT INTO FUNCTION
+Function-based inserts are not supported:
+```sql
+INSERT INTO FUNCTION file('file.parquet') SELECT ...;
+```
+
+### WITH ... AS Subquery Aliases
+Subquery aliases in FROM clauses with keyword `AS`:
+```sql
+SELECT * FROM (SELECT 1 x) AS alias;
+```
+
+### String Concatenation Operator ||
+The `||` operator in some contexts:
+```sql
+SELECT currentDatabase() || '_test' AS key;
+```
+
+### MOD/DIV Operators
+The MOD and DIV keywords as operators:
+```sql
+SELECT number MOD 3, number DIV 3 FROM ...;
+```
+
+### Reserved Keyword Handling
+Keywords like `LEFT`, `RIGHT` used as table aliases:
+```sql
+SELECT * FROM numbers(10) AS left RIGHT JOIN ...;
+```
+
+### Parameterized Settings
+Settings with `$` parameters:
+```sql
+SET param_$1 = 'Hello';
+```
+
+### Incomplete CASE Expression
+CASE without END:
+```sql
+SELECT CASE number  -- missing END
+```
+
+## Explain Output Issues
+
+These require changes to `internal/explain/`:
+
+### Double Equals (==) Operator
+The `==` operator creates extra nested equals/tuple nodes:
+```sql
+SELECT value == '127.0.0.1:9181'
+```
+Expected: `Function equals` with `Identifier` and `Literal`
+Got: Nested `Function equals` with extra `Function tuple`
+
+### CreateQuery Spacing
+Some ClickHouse versions output extra space before `(children`:
+```
+CreateQuery d1  (children 1)  -- two spaces
+CreateQuery d1 (children 1)   -- one space (our output)
+```
+
+### Server Error Messages in Expected Output
+Some test expected outputs include trailing messages:
+```
+The query succeeded but the server error '42' was expected
+```
+These are not part of the actual EXPLAIN output.
+
+## Lower Priority
+
+### DateTime64 with Timezone
+Type parameters with string timezone:
+```sql
+DateTime64(3,'UTC')
+```
+
+### Complex Type Expressions
+Nested type expressions in column definitions:
+```sql
+CREATE TABLE t (c LowCardinality(UUID));
+```
+
+### Parameterized Views
+View definitions with parameters:
+```sql
+CREATE VIEW v AS SELECT ... WHERE x={parity:Int8};
+```
+
+## Testing Notes
+
+Run tests with timeout to catch infinite loops:
+```bash
+go test ./parser -timeout 5s -v
+```
+
+Count test results:
+```bash
+go test ./parser -timeout 5s -v 2>&1 | grep -E 'PASS:|SKIP:' | cut -d':' -f1 | sort | uniq -c
+```
+
+View explain mismatches:
+```bash
+go test ./parser -timeout 5s -v 2>&1 | grep -A 30 "TODO: Explain output mismatch" | head -100
+```