Add performance test for simple seq scans.

Fixes https://github.com/zenithdb/zenith/issues/831

test_runner/performance/test_small_seqscans.py

@@ -0,0 +1,43 @@
+# Test sequential scan speed
+#
+# The test table is large enough (3-4 MB) that it doesn't fit in the compute node
+# cache, so the seqscans go to the page server. But small enough that it fits
+# into memory in the page server.
+from contextlib import closing
+from fixtures.zenith_fixtures import ZenithEnv
+from fixtures.log_helper import log
+from fixtures.benchmark_fixture import MetricReport, ZenithBenchmarker
+
+pytest_plugins = ("fixtures.zenith_fixtures", "fixtures.benchmark_fixture")
+
+
+def test_small_seqscans(zenith_simple_env: ZenithEnv, zenbenchmark: ZenithBenchmarker):
+    env = zenith_simple_env
+    # Create a branch for us
+    env.zenith_cli(["branch", "test_small_seqscans", "empty"])
+
+    pg = env.postgres.create_start('test_small_seqscans')
+    log.info("postgres is running on 'test_small_seqscans' branch")
+
+    # Open a connection directly to the page server that we'll use to force
+    # flushing the layers to disk
+    psconn = env.pageserver.connect()
+    pscur = psconn.cursor()
+
+    with closing(pg.connect()) as conn:
+        with conn.cursor() as cur:
+            cur.execute('create table t (i integer);')
+            cur.execute('insert into t values (generate_series(1,100000));')
+
+            # Verify that the table is larger than shared_buffers
+            cur.execute('''
+            select setting::int * pg_size_bytes(unit) as shared_buffers, pg_relation_size('t') as tbl_ize
+            from pg_settings where name = 'shared_buffers'
+            ''')
+            row = cur.fetchone()
+            log.info(f"shared_buffers is {row[0]}, table size {row[1]}")
+            assert int(row[0]) < int(row[1])
+
+            with zenbenchmark.record_duration('run'):
+                for i in range(1000):
+                    cur.execute('select count(*) from t;')