阶段2（上）：后端专题 Go/Gin/PostgreSQL/MySQL/Redis/MQ/gRPC 的课程、企业场景、面试题和可运行示例

2026-10-23 15:24:57 +08:00
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+# requires: pymysql
+"""对比 LIMIT offset 深分页和游标（keyset）分页。
+
+运行：MYSQL_URL=mysql://user:pass@127.0.0.1:3306/db python paginate.py
+没有设置 MYSQL_URL 时只打印提示并正常退出。会创建并删除临时表 demo_page_orders。
+"""
+import os
+import sys
+import time
+from urllib.parse import urlparse
+
+import pymysql
+
+TABLE = "demo_page_orders"
+ROWS = 100_000
+PAGE = 20
+
+
+def connect(url: str) -> pymysql.connections.Connection:
+    u = urlparse(url)
+    return pymysql.connect(host=u.hostname, port=u.port or 3306, user=u.username, password=u.password or "",
+                           database=u.path.lstrip("/"), autocommit=True)
+
+
+def seed(cur) -> None:
+    cur.execute(f"DROP TABLE IF EXISTS {TABLE}")
+    cur.execute(f"""CREATE TABLE {TABLE} (
+        id BIGINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
+        user_id INT NOT NULL,
+        created_at DATETIME NOT NULL,
+        note VARCHAR(100) NOT NULL,
+        KEY idx_user_time_id (user_id, created_at, id)
+    ) ENGINE = InnoDB""")
+    batch = []
+    for i in range(ROWS):
+        batch.append((1, f"note-{i}"))  # 全部属于 user 1，便于构造深分页
+        if len(batch) == 5000:
+            cur.executemany(f"INSERT INTO {TABLE} (user_id, created_at, note) VALUES (%s, '2026-01-01', %s)", batch)
+            batch.clear()
+    # 时间按 id 递增；让每 3 行共用一个时间，演示“时间相同按 id 排序”
+    cur.execute(f"UPDATE {TABLE} SET created_at = '2026-01-01' + INTERVAL (id DIV 3) SECOND")
+    cur.execute(f"ANALYZE TABLE {TABLE}")
+    cur.fetchall()
+
+
+def offset_page(cur, offset: int) -> list[tuple]:
+    cur.execute(f"SELECT id, created_at, note FROM {TABLE} WHERE user_id = 1 "
+                f"ORDER BY created_at DESC, id DESC LIMIT %s OFFSET %s", (PAGE, offset))
+    return list(cur.fetchall())
+
+
+def deferred_join_page(cur, offset: int) -> list[tuple]:
+    # 延迟关联：先在覆盖索引上找出 id，再回表取这一页的整行
+    cur.execute(f"""SELECT o.id, o.created_at, o.note FROM {TABLE} o
+        JOIN (SELECT id FROM {TABLE} WHERE user_id = 1 ORDER BY created_at DESC, id DESC LIMIT %s OFFSET %s) t
+        ON o.id = t.id ORDER BY o.created_at DESC, o.id DESC""", (PAGE, offset))
+    return list(cur.fetchall())
+
+
+def keyset_page(cur, after: tuple | None) -> list[tuple]:
+    if after is None:
+        cur.execute(f"SELECT id, created_at, note FROM {TABLE} WHERE user_id = 1 "
+                    f"ORDER BY created_at DESC, id DESC LIMIT %s", (PAGE,))
+    else:
+        created_at, last_id = after
+        cur.execute(f"SELECT id, created_at, note FROM {TABLE} WHERE user_id = 1 "
+                    f"AND (created_at < %s OR (created_at = %s AND id < %s)) "
+                    f"ORDER BY created_at DESC, id DESC LIMIT %s", (created_at, created_at, last_id, PAGE))
+    return list(cur.fetchall())
+
+
+def timed(fn, *args) -> tuple[float, list[tuple]]:
+    t = time.perf_counter()
+    rows = fn(*args)
+    return (time.perf_counter() - t) * 1000, rows
+
+
+def main() -> int:
+    url = os.environ.get("MYSQL_URL")
+    if not url:
+        print("未设置 MYSQL_URL，跳过（示例：mysql://app:app@127.0.0.1:3306/learn）")
+        return 0
+    conn = connect(url)
+    cur = conn.cursor()
+    try:
+        seed(cur)
+        offset = ROWS - 1000  # 很深的一页
+        ms1, a = timed(offset_page, cur, offset)
+        ms2, b = timed(deferred_join_page, cur, offset)
+        # 游标分页：拿到上一页最后一行 (created_at, id)，直接从那里继续
+        prev = offset_page(cur, offset - PAGE)[-1]
+        ms3, c = timed(keyset_page, cur, (prev[1], prev[0]))
+        assert a == b == c, "三种写法结果应该一致"
+        print(f"OFFSET {offset}: {ms1:.1f} ms；延迟关联: {ms2:.1f} ms；游标分页: {ms3:.1f} ms（耗时与机器有关，看相对差异）")
+        # 游标分页逐页翻完不重不漏
+        seen, after = 0, None
+        while True:
+            rows = keyset_page(cur, after)
+            if not rows:
+                break
+            seen += len(rows)
+            after = (rows[-1][1], rows[-1][0])
+            if seen >= 2000:
+                break
+        assert seen == 2000
+        print("游标分页连续翻页正确")
+        return 0
+    finally:
+        cur.execute(f"DROP TABLE IF EXISTS {TABLE}")
+        conn.close()
+
+
+if __name__ == "__main__":
+    sys.exit(main())