diff --git a/lib/sql_generator.js b/lib/sql_generator.js
index 63a082a..a6b0076 100644
--- a/lib/sql_generator.js
+++ b/lib/sql_generator.js
@@ -34,11 +34,17 @@ function grainOf(colMeta) {
     // parts and chr(30) stands in for NULL, so ('a', NULL) cannot collide with
     // (NULL, 'a') and a NULL stays distinct from an empty string — a collision
     // would silently merge two groups into one indexed row.
-    const key = (pfx = '') => `concat_ws(chr(31), ${[
+    //
+    // md5 of that, rather than the concatenation itself, because the key is an
+    // opaque handle -- nothing reads it but table.update() and table.remove().
+    // The raw form averaged 233 chars on a 24-column grain and, being unique per
+    // row, defeated Arrow's dictionary encoding: 65.6 MB of a 109 MB payload,
+    // more than every other column combined. 128 bits keeps collisions unreachable.
+    const key = (pfx = '') => `md5(concat_ws(chr(31), ${[
         ...cols.map(c => `COALESCE(${pfx}${q(c)}::text, chr(30))`),
         `${pfx}pf_iter`,
         `${pfx}pf_logid::text`
-    ].join(', ')})`;
+    ].join(', ')}))`;
 
     const groupCols = (pfx = '') => [...cols.map(c => `${pfx}${q(c)}`), `${pfx}pf_iter`, `${pfx}pf_logid`];