diff options
Diffstat (limited to 'src/mongo/db/query/query_knobs.idl')
| -rw-r--r-- | src/mongo/db/query/query_knobs.idl | 19 |
1 files changed, 19 insertions, 0 deletions
diff --git a/src/mongo/db/query/query_knobs.idl b/src/mongo/db/query/query_knobs.idl index fcf9b9e5c86..dc30b285ebb 100644 --- a/src/mongo/db/query/query_knobs.idl +++ b/src/mongo/db/query/query_knobs.idl @@ -135,6 +135,25 @@ server_parameters: validator: gte: 0 + internalQueryEnumerationPreferLockstepOrEnumeration: + description: "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." + set_at: [ startup, runtime ] + cpp_varname: "internalQueryEnumerationPreferLockstepOrEnumeration" + cpp_vartype: AtomicWord<bool> + default: false + internalQueryEnumerationMaxOrSolutions: description: "How many solutions will the enumerator consider at each OR?" set_at: [ startup, runtime ] |