annagobova commented on code in PR #10983:
URL: https://github.com/apache/ignite/pull/10983#discussion_r1358121135
##########
docs/_docs/SQL/sql-calcite.adoc:
##########
@@ -241,3 +228,150 @@ Below are the data types supported by the Calcite-based
SQL engine:
|`java.lang.Object`
|===
+
+== Optimizer hints [[hints]]
+
+The query optimizer does its best to build the fastest excution plan. But it
is a far way to create an optimizer which
+is the most effective for each case. You may have better knowledge of your
data design, application design or data
+distribution in the cluster. The SQL hints can help the optimizer to make
optimizations more rationally or build
+execution plan faster.
+
+[NOTE]
+====
+SQL hints are not mandatory to apply and might be skipped in some cases.
+====
+
+=== Hints format
+Sql hints are defined with the special comment +++/*+ HINT */+++ reffered as
_hint block_. Spaces before and after the
+hint name are required. Hint block is placed right after a relation operator.
Usually, after _SELECT_. Several hint
+blocks for one relation operator *are not allowed*.
+
+Example:
+[source, SQL]
+----
+SELECT /*+ NO_INDEX */ T1.* FROM TBL1 where T1.V1=? and T1.V2=?
+----
+
+It is allowed to define several hints for the same relation operator. For
that, hints are separated with comma
+(spaces are optional).
+
+Example:
+[source, SQL]
+----
+SELECT /*+ NO_INDEX, EXPAND_DISTINCT_AGG */ SUM(DISTINCT V1), AVG(DISTINCT V2)
FROM TBL1 GROUP BY V3 WHERE V3=?
+----
+
+==== Hint parameters
+Hint parameters, if required, are placed within brackets after the hint name
and are separated with commas.
+
+Hint parameter can be quoted. Quoted parameter is case sensitive. Quoted and
unquoted parameters *cannot be*
+defined for the same hint.
+
+Example:
+[source, SQL]
+----
+SELECT /*+ FORCE_INDEX(TBL1_IDX2,TBL2_IDX1) */ T1.V1, T2.V1 FROM TBL1 T1, TBL2
T2 WHERE T1.V1 = T2.V1 AND T1.V2 > ? AND T2.V2 > ?;
+
+SELECT /*+ FORCE_INDEX('TBL2_idx1') */ T1.V1, T2.V1 FROM TBL1 T1, TBL2 T2
WHERE T1.V1 = T2.V1 AND T1.V2 > ? AND T2.V2 > ?;
+----
+
+=== Hint scope
+Hints are difined for a relation operator. Usually for _SELECT_. Most of the
hints are 'visible' for their relation
+operators, for following operators, queries and sub-queries. Hint defined in a
sub-query are 'visible' only for
+this sub-subery and its sub-queries. Hint is not 'visible' for a preceding
relation operator if defined after it.
+
+Example:
+[source, SQL]
+----
+SELECT /*+ NO_INDEX(TBL1_IDX2), FORCE_INDEX(TBL2_IDX2) */ T1.V1 FROM TBL1 T1
WHERE T1.V2 IN (SELECT T2.V2 FROM TBL2 T2 WHERE T2.V1=? AND T2.V2=?);
+
+SELECT T1.V1 FROM TBL1 T1 WHERE T1.V2 IN (SELECT /*+ FORCE_INDEX(TBL2_IDX2) */
T2.V2 FROM TBL2 T2 WHERE T2.V1=? AND T2.V2=?);
+----
+
+Note, only the first query has a hint in a case like:
+[source, SQL]
+----
+SELECT /*+ FORCE_INDEX */ V1 FROM TBL1 WHERE V1=? AND V2=?
+UNION ALL
+SELECT V1 FROM TBL1 WHERE V3>?
+----
+
+But *there are exceptions*: hints of engine or optimizer level like
link:#hint_disable_rule[_DISABLE_RULE_] or
+link:#hint_query_engine[_QUERY_ENGINE_]. Such hints should be defined at the
beginning of query and relates to the
+whole query.
+
+=== Hints errors
+The optimizer tries to apply every hint and hint parameter if able. But skips
hint or hint parameter if:
+
+* There is no such supported hint.
+* Required hint parameters are not passed.
+* Hint parameters passed but hint doesn't support any parameter.
+* Hint parameter is mistaken or relates to an unexisting entity like
unexisting index or table.
+* Current hints or current parameters are incompatible with the previous ones
like forcing and disabling of the same index.
Review Comment:
The current hints or current parameters are incompatible with the previous
ones, such as forcing the use and disabling of the same index.
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