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PostgREST INSERT Fails With 42501 Due To Missing SELECT Privilege When Returning Representation

Anonymous inserts via Supabase REST API fail with 42501 permission denied because PostgREST uses INSERT ... RETURNING, which requires SELECT privilege on the table in addition to INSERT. Direct SQL without RETURNING succeeds, leading to misdiagnosis as an RLS issue.

highConfidence 85%Supabase

Origin Analysis

The REST API uses INSERT ... RETURNING to return the inserted row (supabase-js requests return=representation). PostgreSQL requires SELECT privilege on all returned columns for INSERT ... RETURNING, but the anon role has only INSERT privilege, not SELECT. RLS policies are irrelevant to this privilege check.

1. Create table prospects with RLS enabled and INSERT policy for anon; grant only INSERT to anon. 2. From SQL editor, run `SET ROLE anon; INSERT INTO prospects (...) VALUES (...);` - succeeds. 3. From supabase-js, call `supabase.from('prospects').insert({...})` - fails with 42501. 4. Observe error: permission denied for table prospects.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

Revoking SELECT will cause API inserts to fail again. If you grant SELECT, ensure RLS SELECT policies are correctly configured; RLS policies still restrict which rows are visible, but the SELECT privilege itself is required for RETURNING. In environments where anon should not read any data, prefer the client-side returning: 'minimal' instead of granting SELECT. Test after applying.