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2023-03-17-non-prepared-plan-cache.md

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Proposal: Non-Prepared Plan Cache

Background

TP queries can get lots of performance benefits from Plan Cache, but the current TiDB Plan Cache can only support Prepare/Execute Statements. Most of our users are still using general(non-prepared) queries to access TiDB instead of Prepare/Execute. So if Plan Cache can support general queries, these users can gain some performance benefits as well. Some other mature Databases have already supported this feature: 1) Oracle and 2) OceanBase.

Overview

non-prepared-plan-cache

As the left part of the above diagram show, Plan Cache can be split into 2 parts logically:

  1. The Frontend: this part is used to deal with different protocols (binary or text), and prepare all arguments required by the backend (SQL, AST, Normalized Parameters, etc.)
  2. The Backend: find an available cached Plan from the cache or generate a new Plan for the input arguments

And more specifically, the backend can be abstracted as a function GetPlan(SQL, AST, Params), and it works as below:

  1. Look up the Plan Cache to find a cached plan for the input SQL;
  2. If it can find an available plan, fill this plan with the input parameters, and return the plan; (cache hit)
  3. If not, pass (AST, Params) to the optimizer to generate a new plan, return this plan, and put this plan into the cache; (cache miss)

To support Non-Prepared Plan Cache, logically we can re-use the whole backend, and just update the frontend a little:

  1. Check whether the current query is supported by Non-Prepared Plan Cache;
  2. If not, just fall back to the normal optimization code path;
  3. If yes:
    1. Normalize this query and prepare all arguments (SQL, AST, Params);
    2. Pass these arguments to the Plan Cache backend to get a Plan;

Details

Scope & Limitations

Currently, TiDB only caches one plan for a parameterized query.

For example, the queries SELECT * FROM t WHERE a < 1 and SELECT * FROM t WHERE a < 100000 share the same parameterized form, SELECT * FROM t WHERE a < ?, and thus share the same plan.

Due to the preceding risks and the fact that the execution plan cache only provides significant benefits for simple queries (if a query is complex and takes a long time to execute, using the execution plan cache might not be very helpful), TiDB has strict restrictions on the scope of non-prepared plan cache. The restrictions are as follows:

  1. Queries or plans that are not supported by the Prepared plan cache are also not supported by the non-prepared plan cache.
  2. Currently, only point get or range queries on a single table that contain Scan, Selection, or Projection operators are supported, such as SELECT * FROM t WHERE a < 10 AND b in (1, 2).
  3. Queries that contain complex operators such as Agg, Limit, Window, or Sort are not supported.
  4. Queries that contain non-range query conditions are not supported, such as:
  5. LIKE is not supported, such as c LIKE 'c%'.
  6. + operation is not supported, such as a+1 < 2.
  7. Queries that filter on columns of JSON, ENUM, SET, or BIT type are not supported, such as SELECT * FROM t WHERE json_col = '{}'.
  8. Queries that filter on NULL values are not supported, such as SELECT * FROM t WHERE a is NULL.
  9. Queries with more than 50 parameters after parameterization are not supported, such as SELECT * FROM t WHERE a in (1, 2, 3, ... 51).
  10. Queries that access partitioned tables, virtual columns, temporary tables, views, or memory tables are not supported, such as SELECT * FROM INFORMATION_SCHEMA.COLUMNS, where COLUMNS is a TiDB memory table.
  11. Queries with hints, subqueries, or locks are not supported.
  12. DML statements are not supported.

We'll solve these limitations step by step later on.

Separated Cache

Prepared Plan Cache and Non-Prepared Plan Cache are totally separated, cached plans in them do not affect each other.

Cacheable Check

We implement a cacheable checker to check whether a query can be supported by Non-Prepared Plan Cache(cacheable_checker). If a query is not supported, we can just fall back to the normal optimization code path.

Parameterization

Plan Cache uses parameterized SQL as the key, so we need a way that can parameterize any SQL fast. For example, parameterize select * from t where a<10 to select * from t where a<?; Currently, we implement this on AST:

  1. First, we iterate the AST and replace all constant values with ?;
  2. Then, we call AST.Restore to get the parameterized SQL;

New Variables

Introduce 2 variables below to control Non-Prepared Plan Cache behaviors:

  • tidb_enable_non_prepared_plan_cache: indicate whether to enable this feature;
  • tidb_non_prepared_plan_cache_size: indicate the cache size;