diff options
Diffstat (limited to 'src/mongo/db/query/query_planner_params.h')
| -rw-r--r-- | src/mongo/db/query/query_planner_params.h | 17 |
1 files changed, 17 insertions, 0 deletions
diff --git a/src/mongo/db/query/query_planner_params.h b/src/mongo/db/query/query_planner_params.h index 4b8a5f54904..806456f0508 100644 --- a/src/mongo/db/query/query_planner_params.h +++ b/src/mongo/db/query/query_planner_params.h @@ -103,6 +103,23 @@ struct QueryPlannerParams { // Set this to track the most recent timestamp seen by this cursor while scanning the oplog. TRACK_LATEST_OPLOG_TS = 1 << 12, + + // Instruct the plan enumerator to enumerate contained $ors in a special order. $or + // enumeration can generate an exponential number of plans, and is therefore limited at some + // arbitrary cutoff controlled by a parameter. When this limit is hit, the order of + // enumeration is important. For example, a query like the following has a "contained $or" + // (within an $and): + // {a: 1, $or: [{b: 1, c: 1}, {b: 2, c: 2}]} + // For this query if there are indexes a_b={a: 1, b: 1} and a_c={a: 1, c: 1}, the normal + // enumeration order would output assignments [a_b, a_b], [a_c, a_b], [a_b, a_c], then [a_c, + // a_c]. This flag will instruct the enumerator to instead prefer a different order. It's + // hard to summarize, but perhaps the phrases "lockstep enumeration", "simultaneous + // advancement", or "parallel iteration" will help the reader. The effect is to give earlier + // enumeration to plans which use the same index of alternative across all branches. In this + // order, we would get assignments [a_b, a_b], [a_c, a_c], [a_c, a_b], then [a_b, a_c]. This + // is thought to be helpful in general, but particularly in cases where all children of the + // $or use the same fields and have the same indexes available, as in this example. + ENUMERATE_OR_CHILDREN_LOCKSTEP = 1 << 13, }; // See Options enum above. |