diff options
Diffstat (limited to 'src/mongo/db/query/query_knobs.h')
| -rw-r--r-- | src/mongo/db/query/query_knobs.h | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/src/mongo/db/query/query_knobs.h b/src/mongo/db/query/query_knobs.h index ed4304de4ae..cc28b097c96 100644 --- a/src/mongo/db/query/query_knobs.h +++ b/src/mongo/db/query/query_knobs.h @@ -81,6 +81,21 @@ extern AtomicDouble internalQueryCacheEvictionRatio; // How many indexed solutions will QueryPlanner::plan output? extern AtomicInt32 internalQueryPlannerMaxIndexedSolutions; +// If set to true, instructs 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 choice 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. +extern AtomicBool internalQueryEnumerationPreferLockstepOrEnumeration; + // How many solutions will the enumerator consider at each OR? extern AtomicInt32 internalQueryEnumerationMaxOrSolutions; |