From 8c4d31f036f1389b1a7aae0658bcc09ed800510c Mon Sep 17 00:00:00 2001 From: wenyihu3 Date: Tue, 28 Jun 2022 15:43:13 -0400 Subject: [PATCH] rfc: add rfc for invisible index feature This commit adds an RFC for the invisible index feature. Related issue: https://github.com/cockroachdb/cockroach/issues/72576, https://github.com/cockroachdb/cockroach/issues/82363 Release justification: low risk to the existing functionality; this commit just adds rfc. Release Note: none --- docs/RFCS/20220628_invisible_index.md | 428 ++++++++++++++++++++++++++ 1 file changed, 428 insertions(+) create mode 100644 docs/RFCS/20220628_invisible_index.md diff --git a/docs/RFCS/20220628_invisible_index.md b/docs/RFCS/20220628_invisible_index.md new file mode 100644 index 000000000000..bb55f13a8553 --- /dev/null +++ b/docs/RFCS/20220628_invisible_index.md @@ -0,0 +1,428 @@ +- Feature Name: Invisible Index +- Status: in-progress +- Start Date: 2022-06-28 +- Authors: Wenyi Hu +- RFC PR: https://github.com/cockroachdb/cockroach/pull/83531 +- Cockroach Issue: https://github.com/cockroachdb/cockroach/issues/72576, + https://github.com/cockroachdb/cockroach/issues/82363 + +# Summary + +An invisible index is an index that is maintained up-to-date but is ignored by +the optimizer unless explicitly selected with [index +hinting](https://www.cockroachlabs.com/docs/v22.1/table-expressions#force-index-selection) +or for constraint purposes. + +The main purpose of this RFC is to introduce the feature, document +implementation decisions, and propose a technical design. + +# Motivation: use cases + +### 1. Roll out new indexes with more confidence. +When you create a new index, all queries are able to pick it which could have +an immediate effect on the workload. Currently, some users with large +production scales are concerned about the impact of introducing new indexes and +potentially affecting their applications significantly. + +With invisible indexes, you can introduce the index as invisible first. In a new +session, you could give a workout and observe the impact of the new index by +turning `optimizer_use_not_visible_indexes` on or with index hinting. If this +index does turn out to be useful, you can then change this index to be visible +in your database. + +Note that this allows us to see the impact more safely; the maintenance cost +associated with an index during inserts, upserts, updates, or delete is still +needed. + +### 2. Drop indexes with less risk. +A question that comes up frequently about indexes is whether an index is +actually useful for queries or if it is just sitting around and wasting +maintenance cost. Currently, the only way you can test this is by dropping the +index and then recreating it if the index turns out to be useful. However, when +the table gets large, recreating the index can become really expensive. + +With invisible indexes, you can mark the index as invisible first, wait for a +few weeks to measure the impact, and then drop the index if no drop in +performance is observed. If the index turns out to be needed, you can easily +change the index back to visible without the cost of rebuilding an index. + +Note that using an invisible index reduces the risk associated with dropping the +index but not with no risks. First, just because an index is not used during +this observation period, this does not mean it will not be used in the future. +Second, invisible indexes are still used behind the scene by the optimizer for +any constraint check and maintenance purposes (more details below). In that +case, you cannot expect the database to behave in the exact same way as dropping +an index. + +### 3. Debugging. +If queries suddenly start using an index unexpectedly and is causing performance +issues, you can change the index to be invisible as a short term solution. You +can then investigate what the problem might be using +`optimizer_use_not_visible_indexes` or index hinting in a new session. Once the +issue has been solved, the index can be made visible again. + +### 4. Make indexes only available to specific queries. +If you know certain queries have problems and creating an index would help, you +can use invisible index and make this index available only to queries you want +with index hinting. In this way you can leave the rest of your application +unaffected. + +# Implementation Decisions +### Conclusion: +- Users can create an invisible index or change an existing index to be invisible. +- By default, indexes are visible. +- Primary indexes cannot be invisible. +- Constraints cannot be created with invisible indexes. Creating unique or foreign key constraints with invisible indexes is not supported. +- Partial invisible indexes or inverted invisible indexes are both supported. The behavior is as expected. +- Queries can be instructed to use invisible indexes explicitly through [index + hinting](https://www.cockroachlabs.com/docs/v22.1/table-expressions#force-index-selection). +- Session variable, `optimizer_use_not_visible_indexes`, can be set to true to tell the optimizer to treat invisible indexes as they are visible. By default, `optimizer_use_not_visible_indexes` is set to false. + +The following points are where things might be unexpected as making an index invisible is not exactly the same as dropping the index. +- Force index or index hinting with invisible index is allowed and will override the invisible index feature. + - If the index is dropped instead, this will throw an error. +- Invisible indexes will be treated as visible while policing unique or foreign key constraints. In other words, we will temporarily disable the invisible index feature during any constraint check. + - If the index is dropped, the query plan for constraint check could be different and lead to a full table scan. + +### 1. What types of indexes can be invisible? +Primary indexes cannot be invisible. Any secondary indexes including unique +indexes can be invisible. + +In [MySQL](https://dev.mysql.com/doc/refman/8.0/en/invisible-indexes.html), all +indexes other than primary keys can be invisible. In MySQL, a table with no +explicit primary key will use the first unique index on NOT NULL columns as an +implicit primary key. Implicit and explicit primary indexes are both not allowed +to be invisible. However, a table with no explicit primary key creates a new +rowid column in CRDB, and creating a unique index on a null column will not +change the primary key. + +### 2. Can constraints be invisible? Can constraints be created with invisible indexes? +No. Constraints cannot be invisible, and they cannot be created with invisible +indexes. Having invisible constraint means that the constraint can be on and off +at different times. Since constraint is an insert-time enforcement, allowing +invisible constraint could lead to corrupted indexes. + +One might think creating unique constraints with invisible indexes is similar to +creating unique constraints without indexes (which is something CRDB is +currently supporting). But they have very different semantic meanings. First, +creating a constraint without index is not user-friendly and was created for +multi-tenant testing purposes. Second, creating a constraint with an invisible +index is still an index but just ignored by the optimizer. + +Overall, only indexes can be invisible. Creating a unique constraint or a +foreign key constraint with invisible indexes will not be supported. This leads +to an issue with the parser. This behavior aligns with MySQL. + +This leads to another issue in the parser; creating an invisible unique index +inside a `CREATE TABLE` definition is supported by the grammar rule, but the +parser will throw an error. This is because the parser is doing a round trip in +`pkg/testutils/sqlutils/pretty.go`. In sql.y, creating a unique index in a +`CREATE TABLE` statement returns a new structure +`tree.UniqueConstraintTableDef`. However, creating unique constraints with not +visible indexes is not supported by the parser. When the parser does a round +trip for the following statement, it formats it to a pretty statement using the +unique constraint definition. But the parser does not support unique constraint +with not visible index syntax. So it will fail while parsing the pretty +statement. Since logictest also performs a roundtrip check in +`pkg/sql/logictest/logic.go`, logictest would also fail. But creating a unique +index inside a `CREATE TABLE` definition will still work in a cluster. This is a +known issue. See more details in +https://github.com/cockroachdb/cockroach/pull/65825. + +### 3. Should Force Index with invisible index be allowed? +Using index hinting with invisible indexes is allowed and is part of the feature +design. Although this may lead to different behavior with the index being +dropped, this offers more flexibility with the feature. For example, the +fallback of some queries might be a full table scan and may be too expensive. + +In MySQL, index hinting with invisible indexes +[errors](https://dev.mysql.com/doc/refman/8.0/en/invisible-indexes.html). +Instead, MySQL supports index hinting with the session variable +[optimizer_use_not_visible_indexes](https://dev.mysql.com/doc/refman/8.0/en/switchable-optimizations.html#optflag_use-invisible-indexes). +Users can instruct queries to use invisible indexes by setting this session +variable to true for only specific queries. + +### 4. Are invisible indexes still maintained and up-to-date? +Yes. Just like any other indexes, an invisible index consumes maintenance cost +and resources. Regardless of visibility, indexes are maintained up-to-date with +insert, delete, upsert, and update. + +This behavior aligns with [MySQL](https://dev.mysql.com/doc/refman/8.0/en/invisible-indexes.html). + +### 5. Are unique constraints with invisible indexes still in effect? +Regardless of index visibility, unique indexes still prevent checks for +duplicate values when inserting or updating data. Creating a foreign key +constraint requires unique indexes or constraints on the parent table. Foreign +key constraints are still enforced even if the unique indexes on the parent +table become invisible; if a column in a child table is referencing another +column in the parent table, then this value in the parent table must exist. + +This behavior aligns with [MySQL](https://dev.mysql.com/doc/refman/8.0/en/invisible-indexes.html). + +### 6. Scope of Invisibility: to what extent should the optimizer ignore invisible indexes? Should constraint check use or ignore invisible indexes? +Consider the following situation. Creating a child table requires a parent table +to have a unique index on the FK columns. What happens if the unique index is +invisible here? What happens if this unique index is changed to invisible after +the child table has been created? Consider another case. What happens if INSERT +ON CONFLICT is performed on an invisible unique index? + +The first option would be to ignore the invisible index completely. However, +this means that when insert on the child table may require a full table scan to +police the foreign key check. The same situation applies if a parent table +performs delete or update, or if a child table performs insert, upsert, or +update. This would not only lead to performance issues; having a unique +constraint was necessary to create the child table or to perform INSERT ON +CONFLICT. If the index becomes invisible, does it really make sense to allow +these operations? Overall, this option is not viable. + +The second option would be to throw an error when an operation requires a +constraint check using invisible indexes. The justification behind this option +would be if someone wants to test the impact of dropping an index, this would be +the expected behavior. + +However, if someone wants to drop an index, it does not make sense if they still +want to have a foreign key constraint on it or to perform `INSERT ON CONFLICT`. +In addition, this makes this feature much more limited. As described above in +the motivation section, there are other use cases other than testing the impact +of dropping an index. For example, this feature is also helpful for a staged +rollout of a new index. Throwing an error with `INSERT ON CONFLICT` could lead +to confusion. + +The only option left with us is to allow the optimizer to still use invisible +indexes while policing foreign key constraints or unique constraints. This +obviously has some drawbacks; users can no longer expect dropping an index to +behave exactly the same as marking an index as invisible. But we will try our +best to document this well and log messages on occasions where they cannot +expect the same behavior. On the bright side, this should be the more +standardized way based on MySQL and Oracle. + +We should log warning messages on occasions where users cannot expect the invisible index to be equivalent to dropping an index. +We will log this message: +- if users are changing an existing visible index to invisible or if users are dropping an invisible index +- if this invisible index may be used to police constraint check + - when this invisible index is unique + - or when this invisible index is on a child table, and the first column stored by the index is part of the FK constraint. + +**Conclusion** + +The optimizer will treat all invisible indexes as they are visible for any +unique or foreign key constraint purposes. + +### 7. How to observe the impact on invisible indexes? +- SQL statements or queries will have different execution plans. + - You can see this using `EXPLAIN`. + - If you want to know whether invisible indexes are used for constraint check, + you can use `EXPLAIN (VERBOSE)` and check if `disabled not visible index + feature` is set as part of the scan flags. +- Queries or workload will have different performance. + +# Technical Design +## 1. How does the optimizer support this feature? +As discussed above, to fully support the invisible index feature, we need to +ignore the invisible index unless it is used for constraint check or used during +force index. + +First, let’s ignore the part where we need to disable the invisible index +feature and focus on how the optimizer will ignore invisible indexes in general. + +During exploration, the optimizer will explore every possible query plan using +transformation rules. While constructing equivalent memo groups, the optimizer +will enumerate indexes on a given Scan operator’s table using `ForEach` under +`pkg/sql/opt/xform/scan_index_iter.go`. This is where we can hide the index away +from the optimizer. While enumerating every index, the optimizer can check if +the index is invisible and ignore if it is. The optimizer can effectively ignore +the invisible index by blocking the creation of query plans with invisible +indexes. + +Second, let’s think about what happens when force index is used with invisible +index. Force index will override the invisible index feature. We will just need +to check if the flag for force index is set before ignoring invisible indexes +during exploration. + +Third, let’s think about how to disable invisible index features during +constraint check. During Optbuild, we are constructing scan expression on a +given table using `buildScan` under `pkg/sql/opt/optbuilder/select.go`. We can +add a flag to `ScanPrivate` to indicate if this Scan expression was built for a +constraint check. When the factory constructs the scan expression, this flag +will be passed along as a scan operator property. + +When the optimizer enumerates indexes on a given Scan operator under +`pkg/sql/opt/xform/scan_index_iter.go`, the optimizer can then check if the scan +is built for constraint check before ignoring the invisible index. + +### Foreign key constraint check will be needed: + - When a parent table performs an `UPDATE` or `DELETE` operation, FK check on the child table is needed. + - When a child table performs an `INSERT`, `UPSERT`,or `UPDATE` operation, FK check on the parent table is needed. + - There may be different foreign key actions `[UPDATE | DELETE] [ON CASCADE | SET DEFAULT | SET NULL | NO ACTION | RESTRICT| ON CONSTRAINT]`. +### Unique constraint check will be needed: + - When `INSERT [ON CONFLICT DO NOTHING | DO UPDATE SET | ON CONSTRAINT | DISTINCT ON]` + - When `UPSERT`, `UPDATE` + +## 2. Syntax +### a. CREATE INDEX, CREATE TABLE, ALTER INDEX statements +#### Create Index Statements +```sql +CREATE [UNIQUE | INVERTED] INDEX [CONCURRENTLY] [IF NOT EXISTS] [] + ON ( [ASC | DESC] [, ...] ) + [USING HASH] [STORING ( )] + [PARTITION BY ] + [WITH ] [WHERE ] + [VISIBLE | NOT VISIBLE] +``` +- Example + +```sql +CREATE INDEX a ON b.c (d) VISIBLE +CREATE INDEX a ON b.c (d) NOT VISIBLE + +CREATE INDEX a ON b (c) WITH (fillfactor = 100, y_bounds = 50) VISIBLE +CREATE INDEX a ON b (c) WITH (fillfactor = 100, y_bounds = 50) NOT VISIBLE + +CREATE INDEX geom_idx ON t USING GIST(geom) VISIBLE +CREATE INDEX geom_idx ON t USING GIST(geom) NOT VISIBLE + +CREATE UNIQUE INDEX IF NOT EXISTS a ON b (c) WHERE d > 3 VISIBLE +CREATE UNIQUE INDEX IF NOT EXISTS a ON b (c) WHERE d > 3 NOT VISIBLE +``` + +#### Create Table Statements +```sql +CREATE [[GLOBAL | LOCAL] {TEMPORARY | TEMP}] TABLE [IF NOT EXISTS] [table_element_list] [] +``` + +```sql +table_element_list: index_def +[UNIQUE | INVERTED] INDEX [] ( [ASC | DESC] [, ...] + [USING HASH] [{STORING | INCLUDE | COVERING} ( )] + [PARTITION BY ] + [WITH ] [WHERE ] + [VISIBLE | NOT VISIBLE] +``` + +- Example: +```sql +CREATE TABLE a (b INT8, c STRING, INDEX (b ASC, c DESC) STORING (c) VISIBLE) +CREATE TABLE a (b INT8, c STRING, INDEX (b ASC, c DESC) STORING (c) NOT VISIBLE) + +CREATE TABLE a (b INT, UNIQUE INDEX foo (b) WHERE c > 3 VISIBLE) +CREATE TABLE a (b INT, UNIQUE INDEX foo (b) WHERE c > 3 NOT VISIBLE) +``` + +#### ALTER INDEX Statements +```sql +ALTER INDEX [IF EXISTS] [VISIBLE | NOT VISIBLE] +``` + +```sql +ALTER INDEX a@b VISIBLE +ALTER INDEX a@b NOT VISIBLE +``` + +### b. SHOW INDEX Statements +A new column needs to be added to the output of following SQL statements: +```sql +SHOW INDEX FROM (table_name) +SHOW INDEXES FROM(table_name) +SHOW KEYS FROM (table_name) + +SHOW INDEX FROM DATABASE(database_name) +SHOW INDEXES FROM DATABASE (database_name) +SHOW KEYS FROM DATABASE (database_name) +``` + +``` +table_name index_name non_unique seq_in_index column_name direction storing implicit visible +``` + +### c. Tables that store indexes information +A new column needs to be added to the output of `crdb_internal.table_indexes` and `information_schema.statistics`. +`crdb_internal.table_indexes` +``` +descriptor_id descriptor_name index_id index_name index_type is_unique is_inverted is_sharded ***is_visible*** shard_bucket_count created_at +``` + +`information_schema.statistics` +``` +table_catalog table_schema table_name non_unique index_schema index_name seq_in_index column_name COLLATION cardinality direction storing implicit ***is_visible*** +``` + +## d. Alternative Syntax Considered +### a. CREATE INDEX, CREATE TABLE, ALTER INDEX statements +Invisible index feature is introducing four new user facing syntax. Since +PostgreSQL does not support the invisible index feature yet, we will use MySQL +and Oracle as a reference for the standardized syntax. + +The two options that we have discussed are `NOT VISIBLE` and `INVISIBLE`. + +- Reason why `NOT VISIBLE` is good: CRDB currently supports a similar feature, +invisible column feature. And invisible column feature is using `NOT VISIBLE` +for its syntax. If you are wondering about why the invisible column feature chose +`NOT VISIBLE` over `INVISIBLE`, please look at this PR +https://github.com/cockroachdb/cockroach/pull/26644 for more information. +- Reason why `INVISIBLE` is good: MySQL and Oracle both support `INVISIBLE`. + +**Conclusion**: we have decided that being consistent internally with what CRDB +already has is more important than being consistent with other database engines. + +There has been discussion about supporting INVISIBLE as an alias. But this could +lead to more issues: +1. If we support INVISIBLE as an alias for invisible index +feature, we would have to support INVISIBLE as an alias for the invisible column +feature as well. There are some technical issues in the grammar to do that. +2. If users are migrating from other database to CRDB, they would need to rewrite +their SQL anyway. +3. This might lead to confusion when user tries to create invisible columns or + indexes. Overall, supporting `INVISIBLE` as an alias doesn't seem to provide + a large benefit. + + +### b. SHOW INDEX Statements +The three options are `is_hidden`, `is_visible`, and `visible`. + +- Reason why `is_hidden` is good: invisible column feature is using `is_hidden` for [`SHOW COLUMNS`](https://www.cockroachlabs.com/docs/stable/show-columns.html). +- Reason why `visible` is good: this is more consistent what we chose with the first syntax --- VISIBLE | NOT VISIBLE. MySQL is also using [`visible`](https://dev.mysql.com/doc/refman/8.0/en/show-index.html). +- Reason why `is_visible` is bad: less consistent with other columns in [`SHOW INDEX`](https://www.cockroachlabs.com/docs/stable/show-index.html) such as `storing, implicit, non_unique`. + +**Conclusion**: `visible` it is more important to stay consistent with the first user-facing syntax. + +### c. Tables that store indexes information: `crdb_internal.table_indexes` and `information_schema.statistics` +The three options are `is_hidden`, `is_visible`, and `visible`. + +- Reason why `hidden` is good: Invisible column feature uses `hidden` for `table_columns`. +- Reason why `is_visible` is good: MySQL uses `is_visible`. Also, this is more consistent with other columns in `table_indexes`, such as is_unique, is_inverted, is_sharded. +- Reason why `visibility` is good: Oracle uses `visibility`. + +- **Conclusion**: `is_visible` it is more important to stay consistent with the second-user facing syntax. + +### d. Index Descriptor + +We are also introducing another field in the index descriptor (just for internal +use). The options are `Hidden`, `Invisible`, or `NotVisible`. The invisible +column feature is using `Hidden` in the column descriptor. Using visible or +visibility would be odd as well since the default boolean value is false (by +default, index should be visible). + +- **Conclusion**: `NotVisible`. Since we chose `visible` for all invisible index features above, choosing `NotVisible` or `Invisible` here is more consistent. `NotVisible` is preferred here because we are trying to stay away from the keyword `Invisible` to avoid confusion for the first user-facing syntax. + +For more context on how this conclusion was drawn, please see https://github.com/cockroachdb/cockroach/pull/83388 and this RFC PR’s discussion + +# Fine Grained Control of Index Visibility +As of now, the plan is to introduce the general feature of invisible index +first. The design and implementation details for fine-grained control of index +visibility will be discussed in the future. + +Later on, we want to extend this feature and allow a more fine-grained control +of index visibility by introducing the following two features. + +1. Indexes are not restricted to just being visible or invisible; users can + experiment with different levels of visibility. In other words, instead of + using a boolean invisible flag, users can set a float invisible flag between + 0.0 and 100.0. The index would be made invisible only to a corresponding + fraction of queries. Related: + https://github.com/cockroachdb/cockroach/issues/72576#issuecomment-1034301996 + +2. Different sessions of users can set different index visibility. + Related: https://github.com/cockroachdb/cockroach/issues/82363 + +3. We can consider introducing another session variable or another type of + indexes that provides the exact same behaviour as dropping an index.