[Draft][API Design] Query hints, options.

Opened this issue for discussion about API design. Feel free to correct spelling and comment missed parts. It is completely draft and can be changed frequently depending on found ambiguous scenarios.

Sample query

var query = 
   from t1 in table1
   join t2 in table2 on t1.Id equals t2.Id
   select new { t1, t2 }

---

Oracle

https://docs.oracle.com/cd/B13789_01/server.101/b10752/hintsref.htm#30459

namespace LinqToDB.DataProvider.Oracle;

Any query hint

query = query.StatementHint(q => $"LEADING({q.t1} {q.t2}) USE_NL({q.t1}) INDEX({q.t1} emp_emp_id_pk) USE_MERGE({q.t1}) FULL({q.t1})")

Flashback query

https://oracle-base.com/articles/10g/flashback-query-10g

var query = 
   from t1 in table1.AsOfScn(scnNumber)
   join t2 in table2.AsOfTimestamp(timestamp) on t1.Id equals t2.Id
   select new { t1, t2 }

AsOfScn/AsOfTimestamp applied to query should apply AS OF table hint to all tables in query. So the following queries are equivalent:

var query = query.AsOfScn(scnNumber)

var query = 
   from t1 in table1.AsOfScn(scnNumber)
   join t2 in table2.AsOfScn(scnNumber) on t1.Id equals t2.Id
   select new { t1, t2 }

---

PostgreSQL

Looks like for PostgresSQL we can only define locks after select statement: https://www.postgresql.org/docs/current/sql-select.html Consider to add AfterHint which should add text exactly after whole query:

var query = query.AfterHint(q => $"FOR KEY UPDATE OF {q.t1}");

---

MySql

Looks like the same hints as for Oracle https://dev.mysql.com/doc/refman/8.0/en/optimizer-hints.html#optimizer-hints-syntax

---

Informix

Looks like the same hints as for Oracle but divided by comma https://www.ibm.com/developerworks/data/zones/informix/library/techarticle/0502fan/0502fan.html

---

SqLite

Currently found only INDEXED BY injected before table name https://www.tutlane.com/tutorial/sqlite/sqlite-indexed-by

Consider to add IndexedBy extension for ITable<>

var query = 
   from t1 in table1.IndexedBy("index_name")
   join t2 in table2 on t1.Id equals t2.Id
   select new { t1, t2 }

---

SqlServer

https://docs.microsoft.com/en-us/sql/t-sql/queries/hints-transact-sql?view=sql-server-ver15

[igor-tkachev]

We can add three generic methods to support different types of hints

var query =
(
   from t1 in table1.TableHint("...")
   join t2 in table2.JoinHint("...") on t1.Id equals t2.Id
   select new { t1, t2 }
)
.QueryHint("...")

Also, we can add specific API for each provider.

[jogibear9988]

I think it would be usefull if we can specify that a Hint should only be used for a specific provider.