infoschema: refine info cache logic to reduce the impact of DDL on in…

…formation schema cache (#48284) (#48362)

close #48285

pkg/infoschema/cache.go

@@ -99,15 +99,24 @@ func (h *InfoCache) getSchemaByTimestampNoLock(ts uint64) (InfoSchema, bool) {
  // moreover, the most likely hit element in the array is the first one in steady mode
  // thus it may have better performance than binary search
  for i, is := range h.cache {
- if is.timestamp == 0 || (i > 0 && h.cache[i-1].infoschema.SchemaMetaVersion() != is.infoschema.SchemaMetaVersion()+1) {
- // the schema version doesn't have a timestamp or there is a gap in the schema cache
- // ignore all the schema cache equals or less than this version in search by timestamp
- break
+ if is.timestamp == 0 || ts < uint64(is.timestamp) {
+ // is.timestamp == 0 means the schema ts is unknown, so we can't use it, then just skip it.
+ // ts < is.timestamp means the schema is newer than ts, so we can't use it too, just skip it to find the older one.
+ continue
+ }
+ // ts >= is.timestamp must be true after the above condition.
+ if i == 0 {
+ // the first element is the latest schema, so we can return it directly.
+ return is.infoschema, true
  }
- if ts >= uint64(is.timestamp) {
- // found the largest version before the given ts
+ if h.cache[i-1].infoschema.SchemaMetaVersion() == is.infoschema.SchemaMetaVersion()+1 && uint64(h.cache[i-1].timestamp) > ts {
+ // This first condition is to make sure the schema version is continuous. If last(cache[i-1]) schema-version is 10,
+ // but current(cache[i]) schema-version is not 9, then current schema is not suitable for ts.
+ // The second condition is to make sure the cache[i-1].timestamp > ts >= cache[i].timestamp, then the current schema is suitable for ts.
  return is.infoschema, true
  }
+ // current schema is not suitable for ts, then break the loop to avoid the unnecessary search.
+ break
  }
 
  logutil.BgLogger().Debug("SCHEMA CACHE no schema found")

pkg/infoschema/test/cachetest/BUILD.bazel

@@ -8,7 +8,7 @@ go_test(
  "main_test.go",
  ],
  flaky = True,
- shard_count = 6,
+ shard_count = 7,
  deps = [
  "//pkg/infoschema",
  "//pkg/testkit/testsetup",

pkg/infoschema/test/cachetest/cache_test.go

@@ -15,6 +15,7 @@
 package cachetest
 
 import (
+ "fmt"
  "testing"
 
  "github.com/pingcap/tidb/pkg/infoschema"
@@ -211,3 +212,82 @@ func TestReSize(t *testing.T) {
  require.Nil(t, ic.GetByVersion(3))
  require.Equal(t, is4, ic.GetByVersion(4))
 }
+
+func TestCacheWithSchemaTsZero(t *testing.T) {
+ ic := infoschema.NewCache(16)
+ require.NotNil(t, ic)
+
+ for i := 1; i <= 8; i++ {
+ ic.Insert(infoschema.MockInfoSchemaWithSchemaVer(nil, int64(i)), uint64(i))
+ }
+
+ checkFn := func(start, end int64, exist bool) {
+ require.True(t, start <= end)
+ latestSchemaVersion := ic.GetLatest().SchemaMetaVersion()
+ for ts := start; ts <= end; ts++ {
+ is := ic.GetBySnapshotTS(uint64(ts))
+ if exist {
+ require.NotNil(t, is, fmt.Sprintf("ts %d", ts))
+ if ts > latestSchemaVersion {
+ require.Equal(t, latestSchemaVersion, is.SchemaMetaVersion(), fmt.Sprintf("ts %d", ts))
+ } else {
+ require.Equal(t, ts, is.SchemaMetaVersion(), fmt.Sprintf("ts %d", ts))
+ }
+ } else {
+ require.Nil(t, is, fmt.Sprintf("ts %d", ts))
+ }
+ }
+ }
+ checkFn(1, 8, true)
+ checkFn(8, 10, true)
+
+ // mock for meet error There is no Write MVCC info for the schema version
+ ic.Insert(infoschema.MockInfoSchemaWithSchemaVer(nil, 9), 0)
+ checkFn(1, 7, true)
+ checkFn(8, 9, false)
+ checkFn(9, 10, false)
+
+ for i := 10; i <= 16; i++ {
+ ic.Insert(infoschema.MockInfoSchemaWithSchemaVer(nil, int64(i)), uint64(i))
+ checkFn(1, 7, true)
+ checkFn(8, 9, false)
+ checkFn(10, 16, true)
+ }
+ require.Equal(t, 16, ic.Size())
+
+ // refill the cache
+ ic.Insert(infoschema.MockInfoSchemaWithSchemaVer(nil, 9), 9)
+ checkFn(1, 16, true)
+ require.Equal(t, 16, ic.Size())
+
+ // Test more than capacity
+ ic.Insert(infoschema.MockInfoSchemaWithSchemaVer(nil, 17), 17)
+ checkFn(1, 1, false)
+ checkFn(2, 17, true)
+ checkFn(2, 20, true)
+ require.Equal(t, 16, ic.Size())
+
+ // Test for there is a hole in the middle.
+ ic = infoschema.NewCache(16)
+
+ // mock for restart with full load the latest version schema.
+ ic.Insert(infoschema.MockInfoSchemaWithSchemaVer(nil, 100), 100)
+ checkFn(1, 99, false)
+ checkFn(100, 100, true)
+
+ for i := 1; i <= 16; i++ {
+ ic.Insert(infoschema.MockInfoSchemaWithSchemaVer(nil, int64(i)), uint64(i))
+ }
+ checkFn(1, 1, false)
+ checkFn(2, 15, true)
+ checkFn(16, 16, false)
+ checkFn(100, 100, true)
+ require.Equal(t, 16, ic.Size())
+
+ for i := 85; i < 100; i++ {
+ ic.Insert(infoschema.MockInfoSchemaWithSchemaVer(nil, int64(i)), uint64(i))
+ }
+ checkFn(1, 84, false)
+ checkFn(85, 100, true)
+ require.Equal(t, 16, ic.Size())
+}