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Signed-off-by: crazycs520 <crazycs520@gmail.com>

pkg/infoschema/cache.go

@@ -100,14 +100,22 @@ func (h *InfoCache) getSchemaByTimestampNoLock(ts uint64) (InfoSchema, bool) {
 	// thus it may have better performance than binary search
 	for i, is := range h.cache {
 		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 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
 	}