internal/db2: Reduce temp-file I/O on account-filtered /trades queries

CHANGELOG.md

@@ -5,6 +5,12 @@ file. This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Fixed
+- Reduce PostgreSQL temp-file I/O for the `/trades` endpoint when filtered by account, offer, or liquidity pool. The account-filter query pattern was the dominant contributor to slow-query time and replication lag on pubnet. The subquery UNION now uses `UNION ALL` (the two branches are disjoint by protocol invariant — an account cannot match its own offer, a trade matches two distinct offers, and an LP trade has the pool on exactly one side) and each branch now applies the outer `LIMIT` so only the top rows per branch are materialized rather than the entire filtered set.
+
+### DB Schema Migration
+- Add composite index `htrd_by_base_account_op_order` on `history_trades(base_account_id, history_operation_id, "order")` and `htrd_by_counter_account_op_order` on `(counter_account_id, history_operation_id, "order")` to give the planner a density-independent, spill-free plan for account-filtered trade queries. At pubnet scale each index build takes multiple hours; the migration uses `CREATE INDEX CONCURRENTLY` to avoid blocking writes. The existing single-column indexes `htrd_by_base_account` and `htrd_by_counter_account` are retained until follow-up observation confirms the composites cover all usage.
+
 ## 26.0.0
 
 **This release adds support for Protocol 26**

internal/db2/history/trade.go

@@ -204,8 +204,8 @@ func createTradesSQL(page db2.PageQuery, oldestLedger int32, query historyTrades
 			secondSelect = sql.Where("htrd.counter_liquidity_pool_id = ?", query.poolID)
 		}
 
-		firstSelect = appendOrdering(firstSelect, oldestLedger, op, idx, page.Order)
-		secondSelect = appendOrdering(secondSelect, oldestLedger, op, idx, page.Order)
+		firstSelect = appendOrdering(firstSelect, oldestLedger, op, idx, page.Order).Limit(page.Limit)
+		secondSelect = appendOrdering(secondSelect, oldestLedger, op, idx, page.Order).Limit(page.Limit)
 		firstSQL, firstArgs, err := firstSelect.ToSql()
 		if err != nil {
 			return "", nil, errors.Wrap(err, "error building a firstSelect query")
@@ -215,7 +215,13 @@ func createTradesSQL(page db2.PageQuery, oldestLedger int32, query historyTrades
 			return "", nil, errors.Wrap(err, "error building a secondSelect query")
 		}
 
-		rawSQL := fmt.Sprintf("(%s) UNION (%s) ", firstSQL, secondSQL)
+		// UNION ALL is safe here: the two branches filter on base_/counter_ columns of the same
+		// field (account, offer, or liquidity_pool), and stellar-core's invariants guarantee those
+		// columns never hold the same non-null value in a single trade row — an account cannot
+		// match its own offer, a trade matches two distinct offers, and an LP trade has one side
+		// as the pool and the other as an account (so exactly one of base_/counter_liquidity_pool_id
+		// is non-null). This lets PostgreSQL skip the dedup sort UNION would otherwise force.
+		rawSQL := fmt.Sprintf("(%s) UNION ALL (%s) ", firstSQL, secondSQL)
 		args := append(firstArgs, secondArgs...)
 		// Order the final UNION:
 		switch page.Order {

internal/db2/history/trade_test.go

@@ -1,6 +1,7 @@
 package history
 
 import (
+	"strings"
 	"testing"
 
 	"github.com/stellar/go-stellar-sdk/toid"
@@ -41,6 +42,34 @@ func filterByAccount(trades []Trade, account string) []Trade {
 	return result
 }
 
+// TestCreateTradesSQLUsesUnionAll guards against a regression where the account,
+// offer, or liquidity-pool filter variant's subquery UNION is switched back to
+// plain UNION (which forces a server-side dedup sort and was the dominant
+// temp-file I/O source on pubnet). The two branches are disjoint by protocol
+// invariant, so UNION ALL is semantically equivalent and significantly cheaper.
+func TestCreateTradesSQLUsesUnionAll(t *testing.T) {
+	cases := []struct {
+		name  string
+		query historyTradesQuery
+	}{
+		{"account", historyTradesQuery{accountID: 1, tradeType: AllTrades, orderPreserved: true}},
+		{"offer", historyTradesQuery{offerID: 1, tradeType: AllTrades, orderPreserved: true}},
+		{"pool", historyTradesQuery{poolID: 1, tradeType: AllTrades, orderPreserved: true}},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			sql, _, err := createTradesSQL(descPQ, 0, c.query)
+			if err != nil {
+				t.Fatalf("createTradesSQL failed: %v", err)
+			}
+			if !strings.Contains(sql, "UNION ALL") {
+				t.Errorf("generated SQL must use UNION ALL (not UNION) for the %s filter; got: %s", c.name, sql)
+			}
+		})
+	}
+}
+
 func TestSelectTrades(t *testing.T) {
 	tt := test.Start(t)
 	defer tt.Finish()