sqldb: simplify and fixup the existing invoice store schema and queries

This commit attempts to fix some issues with the invoice store's schema that we
couldn't foresee before the implementation was finished. This is safe as the
schema has not been instantiated yet outside of unit tests. Furthermore the
commit updates invoice store SQL queries according to fixes in the schema as
well as to prepare the higher level implementation in the upcoming commits.

sqldb/sqlc/migrations/000002_amp_invoices.up.sql

@@ -1,5 +1,6 @@
--- amp_invoices_payments  
-CREATE TABLE IF NOT EXISTS amp_invoice_payments (
+-- amp_sub_invoices holds all AMP sub-invoices that belong to a given parent
+-- invoice.
+CREATE TABLE IF NOT EXISTS amp_sub_invoices (
     -- The set id identifying the payment. 
     set_id BLOB PRIMARY KEY,
 
@@ -10,26 +11,35 @@ CREATE TABLE IF NOT EXISTS amp_invoice_payments (
     -- Timestamp of when the first htlc for this payment was accepted.
     created_at TIMESTAMP NOT NULL,
 
-    -- If settled, the invoice payment related to this set id.
-    settled_index INTEGER REFERENCES invoice_payments(id),
+    -- When the invoice was settled.
+    settled_at TIMESTAMP,
 
-    -- The invoice id this set id is related to.
-    invoice_id INTEGER NOT NULL REFERENCES invoices(id)
+    -- If settled, the index is set to the current_value+1 of the settle_index
+    -- seuqence in the invoice_sequences table. If not settled, then it is set
+    -- to NULL.
+    settle_index BIGINT,
+
+    -- The id of the parent invoice this AMP invoice belongs to.
+    invoice_id BIGINT NOT NULL REFERENCES invoices(id) ON DELETE CASCADE,
+
+    -- A unique constraint on the set_id and invoice_id is needed to ensure that
+    -- we don't have two AMP invoices for the same invoice.
+    UNIQUE (set_id, invoice_id)
 );
 
-CREATE INDEX IF NOT EXISTS amp_invoice_payments_invoice_id_idx ON amp_invoice_payments(invoice_id);
+CREATE INDEX IF NOT EXISTS amp_sub_invoices_invoice_id_idx ON amp_sub_invoices(invoice_id);
 
 -- amp_invoice_htlcs contains the complementary information for an htlc related 
 -- to an AMP invoice.
-CREATE TABLE IF NOT EXISTS amp_invoice_htlcs (
+CREATE TABLE IF NOT EXISTS amp_sub_invoice_htlcs (
+    -- The invoice id this entry is related to.
+    invoice_id BIGINT NOT NULL REFERENCES invoices(id) ON DELETE CASCADE,
+
     -- The set id identifying the payment this htlc belongs to.
-    set_id BLOB NOT NULL REFERENCES amp_invoice_payments(set_id),
+    set_id BLOB NOT NULL REFERENCES amp_sub_invoices(set_id) ON DELETE CASCADE,
 
     -- The id of the htlc this entry blongs to.
-    htlc_id BIGINT NOT NULL REFERENCES invoice_htlcs(id),
-
-    -- The invoice id this entry is related to.
-    invoice_id INTEGER NOT NULL REFERENCES invoices(id),
+    htlc_id BIGINT NOT NULL REFERENCES invoice_htlcs(id) ON DELETE CASCADE,
 
     -- The root share for this amp htlc.
     root_share BLOB NOT NULL,
@@ -48,7 +58,7 @@ CREATE TABLE IF NOT EXISTS amp_invoice_htlcs (
     preimage BLOB 
 );
 
-CREATE INDEX IF NOT EXISTS amp_htlcs_set_id_idx ON amp_invoice_htlcs(set_id);
-CREATE INDEX IF NOT EXISTS amp_htlcs_invoice_id_idx ON amp_invoice_htlcs(invoice_id);
-CREATE INDEX IF NOT EXISTS amp_htlcs_htlc_id_idx ON amp_invoice_htlcs(htlc_id);
+CREATE INDEX IF NOT EXISTS amp_htlcs_invoice_id_idx ON amp_sub_invoice_htlcs(invoice_id);
+CREATE INDEX IF NOT EXISTS amp_htlcs_set_id_idx ON amp_sub_invoice_htlcs(set_id);
+CREATE INDEX IF NOT EXISTS amp_htlcs_htlc_id_idx ON amp_sub_invoice_htlcs(htlc_id);