Glossary

Project-specific terms and acronyms that appear throughout this codebase. For external concepts (Raft, MVCC, snapshot isolation) refer to the upstream TiDB documentation; this glossary is scoped to vocabulary that the source code itself uses.

Term	Meaning
AST	Abstract syntax tree produced by the parser; types live in pkg/parser/ast/.
BR	Backup & Restore — the CLI tool and library under br/.
Bazel prepare	The make bazel_prepare step that regenerates BUILD.bazel/*.bzl files. Required after Go-source structural changes. See AGENTS.md -> Build Flow.
Cascades	The newer Cascades-style optimizer being developed under pkg/planner/cascades/. The current default optimizer lives in pkg/planner/core/.
Chunk	The columnar in-memory batch format used in execution; defined in pkg/util/chunk/.
Coprocessor	TiKV/TiFlash request type that pushes filters/projections/aggregations down to the storage node. Driven by pkg/store/copr/ and pkg/distsql/.
DDL job	A unit of work for schema change. Submitted by pkg/ddl/job_submitter.go, scheduled by pkg/ddl/job_scheduler.go, executed by pkg/ddl/job_worker.go.
DistSQL	Distributed SQL request layer that builds and dispatches coprocessor requests. Lives in pkg/distsql/.
Domain	The per-TiDB-instance object that owns the schema/infoschema cache, the statistics handle, the bind manager, the slow-query top-N, the runaway controller, and most background workers. Defined in pkg/domain/domain.go.
DXF	Distributed eXecution Framework. Cluster-wide task framework used for distributed ADD INDEX, IMPORT INTO, and statistics jobs. Lives in pkg/dxf/.
Failpoint	The pingcap/failpoint library used for fault injection in tests. Some packages require make bazel-failpoint-enable (or the failpoint-go-test.sh helper) before running tests.
HTAP	Hybrid Transactional/Analytical Processing. TiKV (row store) + TiFlash (columnar replica) serve OLTP and OLAP from the same data.
Infoschema	The in-memory schema snapshot used for query compilation. pkg/infoschema/. Each Domain reload swaps the live infoschema.
Keyspace	Logical multi-tenancy boundary in TiKV. Code lives in pkg/keyspace/ and is referenced via kerneltype in cmd/tidb-server/main.go.
KV	Key-value abstraction over the storage layer. Interfaces in pkg/kv/; TiKV implementation under pkg/store/.
Lightning	TiDB Lightning, the high-throughput data importer. Runtime in lightning/, libraries in pkg/lightning/.
Logical plan / Physical plan	Output of the planner. Logical operators live in pkg/planner/core/operator/logicalop/; physical operators in pkg/planner/core/operator/physicalop/.
MPP	Massively Parallel Processing query execution against TiFlash. Coordinated by pkg/executor/mpp_gather.go and pkg/executor/mppcoordmanager/, dispatched via pkg/store/copr/mpp.go.
PD	Placement Driver — cluster metadata, region scheduling, and timestamp oracle service. External component; TiDB connects via the tikv/client-go client.
Plan cache	LRU cache of compiled prepared-statement plans. Files: pkg/planner/core/plan_cache*.go.
Region	Range of keys served by a TiKV Raft group. TiDB caches region routing in pkg/store/copr/region_cache.go.
Region cache	Client-side cache of region-to-peer routing fetched from PD.
Resource group	SQL-level resource control (RU). Code under pkg/resourcegroup/ and pkg/resourcemanager/.
RU	Request Unit — abstract resource unit used by resource groups.
SCS / standby	Single Cluster Service / standby logic in pkg/standby/ for warm-up and graceful failover.
SQL digest	Normalized hash of a SQL statement; produced by pkg/parser/digester.go. Used for stmt summary, plan capture, and slow log correlation.
Statement summary	In-memory aggregated SQL statistics; persisted to system tables. Lives in pkg/util/stmtsummary/.
TiDB-server	The SQL frontend daemon built from cmd/tidb-server/main.go. The "TiDB" in this repo nearly always means the SQL server, not the cluster.
TiFlash	Columnar storage replica, used for HTAP. Separate process; TiDB talks to it via MPP/coprocessor RPCs.
TiKV	Distributed transactional KV store. Separate process; TiDB connects via tikv/client-go/v2.
TopN	A statistics structure tracking the most frequent values per column/index. Built in pkg/statistics/cmsketch.go/fmsketch.go.
TSO	Timestamp Oracle. PD-issued monotonic timestamps used as transaction commit/start TS.
TTL	Time-to-live job framework that deletes expired rows. pkg/ttl/.
Unistore	In-process mock TiKV used by tests and the --store=unistore mode. pkg/store/mockstore/unistore/.
Upgrade	Bootstrap-time schema/version migration logic. Defined in pkg/session/upgrade_def.go and driven by pkg/session/upgrade_run.go.