Batch concat_ws calls to support tables with 100+ columns

db/queries/queries.go

@@ -555,6 +555,10 @@ func isNumericType(colType string) bool {
 // When allCols is provided, it returns concat_ws('|', col1_expr, col2_expr, ...)
 // with numeric/decimal columns wrapped in trim_scale() to normalize trailing zeros.
 // If allCols is nil/empty, falls back to the table-alias::text whole-row cast.
+//
+// PostgreSQL limits functions to 100 arguments. Since concat_ws uses 1 argument
+// for the separator, at most 99 column expressions fit per call. For wider
+// tables, the expressions are batched into nested concat_ws calls.
 func buildRowTextExpr(tableAlias string, allCols []string, colTypes map[string]string) string {
 	if len(allCols) == 0 {
 		return tableAlias + "::text"
@@ -570,7 +574,29 @@ func buildRowTextExpr(tableAlias string, allCols []string, colTypes map[string]s
 			exprs[i] = fmt.Sprintf("COALESCE(%s::text, '')", qualifiedCol)
 		}
 	}
-	return fmt.Sprintf("concat_ws('|', %s)", strings.Join(exprs, ", "))
+	return concatWSBatched(exprs)
+}
+
+// concatWSBatched produces a concat_ws('|', ...) expression. When len(exprs)
+// exceeds 99 (the max value-arguments per concat_ws call, since the separator
+// takes one slot), it splits the expressions into batches and nests the calls.
+func concatWSBatched(exprs []string) string {
+	const maxArgs = 99 // 100 total - 1 for the separator
+
+	if len(exprs) <= maxArgs {
+		return fmt.Sprintf("concat_ws('|', %s)", strings.Join(exprs, ", "))
+	}
+
+	// Split into batches, wrap each in its own concat_ws, then combine.
+	var batches []string
+	for i := 0; i < len(exprs); i += maxArgs {
+		end := i + maxArgs
+		if end > len(exprs) {
+			end = len(exprs)
+		}
+		batches = append(batches, fmt.Sprintf("concat_ws('|', %s)", strings.Join(exprs[i:end], ", ")))
+	}
+	return fmt.Sprintf("concat_ws('|', %s)", strings.Join(batches, ", "))
 }
 
 func BlockHashSQL(schema, table string, primaryKeyCols []string, mode string, includeLower, includeUpper bool, filter string, allCols []string, colTypes map[string]string) (string, error) {

db/queries/queries_test.go

@@ -12,6 +12,7 @@
 package queries
 
 import (
+	"fmt"
 	"strings"
 	"testing"
 )
@@ -409,3 +410,60 @@ func TestBlockHashSQL(t *testing.T) {
 		})
 	}
 }
+
+func TestConcatWSBatched(t *testing.T) {
+	t.Run("under limit is single concat_ws", func(t *testing.T) {
+		exprs := make([]string, 50)
+		for i := range exprs {
+			exprs[i] = fmt.Sprintf("col%d", i)
+		}
+		result := concatWSBatched(exprs)
+		// Should be a single concat_ws with all 50 expressions
+		if strings.Count(result, "concat_ws(") != 1 {
+			t.Errorf("expected 1 concat_ws call, got: %s", result)
+		}
+	})
+
+	t.Run("at limit of 99 is single concat_ws", func(t *testing.T) {
+		exprs := make([]string, 99)
+		for i := range exprs {
+			exprs[i] = fmt.Sprintf("col%d", i)
+		}
+		result := concatWSBatched(exprs)
+		if strings.Count(result, "concat_ws(") != 1 {
+			t.Errorf("expected 1 concat_ws call for exactly 99 exprs, got: %s", result)
+		}
+	})
+
+	t.Run("100 expressions nests into batches", func(t *testing.T) {
+		exprs := make([]string, 100)
+		for i := range exprs {
+			exprs[i] = fmt.Sprintf("col%d", i)
+		}
+		result := concatWSBatched(exprs)
+		// Should have 3 concat_ws calls: 1 outer + 2 inner (99 + 1)
+		if strings.Count(result, "concat_ws(") != 3 {
+			t.Errorf("expected 3 concat_ws calls for 100 exprs, got %d in: %s",
+				strings.Count(result, "concat_ws("), result)
+		}
+		// Every expression should be present
+		for _, e := range exprs {
+			if !strings.Contains(result, e) {
+				t.Errorf("missing expression %s in result", e)
+			}
+		}
+	})
+
+	t.Run("200 expressions nests into 3 inner batches", func(t *testing.T) {
+		exprs := make([]string, 200)
+		for i := range exprs {
+			exprs[i] = fmt.Sprintf("col%d", i)
+		}
+		result := concatWSBatched(exprs)
+		// 99 + 99 + 2 = 3 inner batches + 1 outer = 4 concat_ws calls
+		if strings.Count(result, "concat_ws(") != 4 {
+			t.Errorf("expected 4 concat_ws calls for 200 exprs, got %d in: %s",
+				strings.Count(result, "concat_ws("), result)
+		}
+	})
+}