[tarantool-patches] Re: [PATCH] sql: rfc for foreign keys

[Vladislav Shpilevoy]

Please, fix markup. On GitHub the document looks weird.

On 27/06/2018 16:11, Nikita Pettik wrote:
> Part of #3271
> ---
> Branch: https://github.com/tarantool/tarantool/tree/np/gh-3271-foreign-keys-rfc
> Issue: https://github.com/tarantool/tarantool/issues/3271
> 
>   doc/rfc/3271-foreign-keys.md | 187 +++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 187 insertions(+)
>   create mode 100644 doc/rfc/3271-foreign-keys.md
> 
> diff --git a/doc/rfc/3271-foreign-keys.md b/doc/rfc/3271-foreign-keys.md
> new file mode 100644
> index 000000000..6af162263
> --- /dev/null
> +++ b/doc/rfc/3271-foreign-keys.md
> @@ -0,0 +1,187 @@
> +# Foreign Keys
> +
> +* **Status**: In progress
> +* **Start date**: 27-06-2018
> +* **Authors**: Nikita Pettik @korablev77 korablev at tarantool.org
> +* **Issues**: [#3271](https://github.com/tarantool/tarantool/issues/3271)
> +
> +## Summary
> +
> +Introduce Foreign Keys (FK) constraints in server.
> +
> +## Background and motivation
> +
> +Used terminology:
> +
> +CREATE TABLE t1 (id1 INT PRIMARY KEY, a INT);
> +CREATE TABLE t2 (id2 INT PRIMARY KEY REFERENECES t1(id1));
> +
> +t1 - parent or referenced table; id1 - referenced columns;
> +t2 - child or referencing table; id2 - referencing columns;
> +
> +Originally, SQLite provides quite specific way of handling FK constraints:
> +any checks are deferred as much as possible. For instance, existence of
> +parent table's existence is verified each time on data manipulation in
> +child space, but not checked on creation of FK constraint itself.
> +The same is for UNIQUE index: according to ANSI referenced columns must
> +make up UNIQUE constraint; otherwise error is raised. But in SQLite
> +presence of such index is tested only when child table is involved in DML.
> +On the other hand it allows us to get rid of checks on indexes/spaces
> +destroying; moreover, it allows us to avoid storing additional dependencies
> +in internal data dictionary holding only space name and extracting fresh
> +pointers to DD by lookup in hash.
> +
> +To be closer to ANSI FK constraints, it was suggested to rework and
> +move them to server DD.
> +
> +## Implementation details
> +
> +1. Prohibit actions with FK constraints which strictly contradict ANSI:
> + - Ban opportunity to create FK constraint on table which doesn't exist
> +   (except for self-references). Since SQLite allows creating
> +   FK constraints only within <CREATE TABLE> declaration (i.e. there is
> +   no separate <ALTER TABLE ADD CONSTRAINT> statement), this step will
> +   deprive us of ability to create circular FK dependencies.
> + - Ban opportunity to drop parent table even if it doesn't violate FK
> +   constraints. Child table must always be dropped before parent.
> + - Ban opportunity to create FK constraints on VIEW.
> +
> +2. To return ability to create circular FK dependecies, we are going to
> +   introduce <ALTER TABLE ADD CONSTRAINT> statement. It compels us to
> +   store somehow created constraints. Hence, new system space to
> +   persist FK constraints is added:
> +
> + - [350, 1, '_constraint', 'memtx', 0, {}, [{'name': 'name', 'type': 'string'}, {
> +      'name': 'parent_id', 'type': 'unsigned'}, {'name': 'child_id', 'type': 'unsigned'},
> +     {'name': 'deferred', 'type': 'boolean'}, {'name': 'match', 'type': 'string'},
> +     {'name': 'on_delete', 'type': 'string'}, {'name': 'on_update', 'type': 'string'},
> +     {'name': 'links', 'type': 'map'}]]
> +
> +In other words, FK constraint is completely described by:
> + - Its name;
> + - Referenced space;
> + - Child space;
> + - Time of resolution (deferred until the end of transaction or not);
> + - Match clause;
> + - ON DELETE and ON UPDATE actions;
> + - Column links;
> +
> +3. Insertion on _constraint space leads to creation of FK constraints.
> +   FK constraints in DD are represented as following structs:
> +
> ++struct fkey_def {
> ++       /** id of child space. */
> ++       uint32_t child_id;
> ++       /** id of parent space. */
> ++       uint32_t parent_id;
> ++       /** Number of fields (links) in this key. */
> ++       uint32_t field_count;
> ++       /** True if constraint checking is deferred till COMMIT. */
> ++       bool is_deferred;
> ++       /** Match condition for foreign key. SIMPLE by default. */
> ++       enum fkey_match match;
> ++       /** ON DELETE action. */
> ++       enum fkey_action on_delete;
> ++       /** ON UPDATE action. */
> ++       enum fkey_action on_update;
> ++       /** Mapping of fields in child to fields in parent. */
> ++       struct field_link *links;
> ++       /** Name of the constraint. */
> ++       char name[0];
> ++};
> +
> ++struct fkey {
> ++       /** Space containing the REFERENCES clause (aka: child). */
> ++       struct space *child;
> ++       /** Space that the key points to (aka: parent). */
> ++       struct space *parent;
> ++       /** Number of fields (links) in this key. */
> ++       uint32_t field_count;
> ++       /** True if constraint checking is deferred till COMMIT. */
> ++       bool is_deferred;
> ++       /** Match condition for foreign key. SIMPLE by default. */
> ++       enum fkey_match match;
> ++       /** Triggers for actions. */
> ++       struct Trigger *on_delete_trigger;
> ++       struct Trigger *on_update_trigger;
> ++       /** Contraints are orginized into list. */
> ++       struct fkey *fkey_next;
> ++       /** Mapping of fields in child to fields in parent. */
> ++       struct field_link *links;
> ++       /** Name of the constraint. */
> ++       char name[0];
> ++};
> +
> +struct fkey_def is used only during creation of struct fkey.
> +Foreign keys will be stored in struct space as two linked lists:
> +
> +@@ -182,7 +182,8 @@ struct space {
> +         */
> +        struct index **index;
> +        /** Foreign key constraints. */
> ++       struct fkey *parent_fkey;
> ++       struct fkey *child_fkey;
> +
> +on_replace_dd_constraint() would create definition of FK constrain
> +from tuple; from def construct struct fkey, and finally add it
> +to parent's list of FK constraints and to child's one.
> +In case of WAL fail, simply remove it from those lists.
> +On drop of FK actions are inverted: FK constraint is removed from
> +parent and child lists and if WAL fails, it will be returned back.
> +
> +4. Alongside with insertion to _constraint, we must check that tuples
> +   which are already in child table don't violate FK constraint under
> +   construction. There several approaches to resolve this problem.
> +   The first and common one is to handle this routine within
> +   on_replace_dd_constraint trigger: create iterators for parent and
> +   child spaces and for each tuple from child space find appropriate
> +   tuple with given FK key in parent space. The question here is how
> +   to process inserted or deleted tuples until FK constraint is committed?
> +   Another solution is to temporary execute this check only if creation
> +   of FK consraint is occurred via SQL: before insertion to _constraint
> +   run VDBE program which will test this condition. Anyway, now it is
> +   impossible to resolve FK constraints (in common sense) without
> +   involving VDBE: we can't run ON DELETE and ON UPDATE triggers directly
> +   from server. The last and the easiest way is to allow creation of
> +   FK constraint only for empty spaces and remove this restriction when
> +   we will be able to run any VDBE code from server.
> +
> +5. Introduce SQL statement to create and drop FK constraints (ANSI syntax):
> +
> +ALTER TABLE <referencing table> ADD CONSTRAINT
> +  <referential constraint name> FOREIGN KEY
> +  <left parent> <referencing columns> <right paren> REFERENCES
> +  <referenced table> [ <referenced columns> ] [ MATCH <match type> ]
> +  [ <referential triggered action> ] [ <constraint check time> ]
> +
> +ALTER TABLE <referencing table> DROP CONSTRAINT <referential constrain name>
> +
> +In terms of our SQL parser:
> +
> +cmd ::= ALTER TABLE fullname(X) ADD CONSTRAINT nm(Z) FOREIGN KEY
> +        LP eidlist(FA) RP REFERENCES nm(T) eidlist_opt(TA) refargs(R)
> +        defer_subclause_opt(D).
> +
> +cmd ::= ALTER TABLE fullname(X) DROP CONSTRAINT nm(Z).
> +
> +Example:
> +
> +ALTER TABLE t1 ADD CONSTRAINT f1 FOREIGN KEY(id, a) REFERENCES t2 (id, b) MATCH FULL;
> +ALTER TABLE t1 DROP CONSTRAINT f1;
> +
> +These statements are going to emit VDBE code to construct appropriate
> +tuple and insert it into _constraint space (and maybe run VDBE program
> +to make sure that all tuples in child space satisfy new FK constraint).
> +The rest of routine will be handled by on_replace_dd_constraint() trigger.
> +
> +6. Refactor SQL routine to operate on new struct fkey instead of
> +   obsolete SQLite struct FKey.
> +
> +## Open questions
> +
> +Should we resolve fields and indexes right after insertion to
> +_constraint (as it happens in other DBs) or defer it until usage of FK
> +(as it occurs in SQLite)? If we chose first way, we would have to
> +implicitly link index to particular index and ban ability to drop such
> +index until all FK constraints are dropped. Such behaviour may be not
> +so obvious for users, but it is used for instance in PostgreSQL and MySQL.
>