what is “planSummary: IDHACK”?

Question

This Query scans only one document and returns only one document. But this is very slow:

2017-05-22T07:13:24.548+0000 I COMMAND [conn40] query databasename.collectionname query: { _id: ObjectId('576d4ce3f2d62a001e84a9b8') } planSummary: IDHACK ntoskip:0 keysExamined:1 docsExamined:1 idhack:1 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:42 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 8009ms
2017-05-22T07:13:24.549+0000 I COMMAND [conn10] query databasename.collectionname query: { _id: ObjectId('576d4db35de5fa001ebdd77a') } planSummary: IDHACK ntoskip:0 keysExamined:1 docsExamined:1 idhack:1 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:42 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 8010ms
2017-05-22T07:13:24.553+0000 I COMMAND [conn47] query databasename.collectionname query: { _id: ObjectId('576d44b7ea8351001ea5fb22') } planSummary: IDHACK ntoskip:0 keysExamined:1 docsExamined:1 idhack:1 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:42 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 8014ms
2017-05-22T07:13:24.555+0000 I COMMAND [conn52] query databasename.collectionname query: { _id: ObjectId('576d457ceb82a0001e205bfa') } planSummary: IDHACK ntoskip:0 keysExamined:1 docsExamined:1 idhack:1 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:42 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 8015ms
2017-05-22T07:13:24.555+0000 I COMMAND [conn41] query databasename.collectionname query: { _id: ObjectId('576d457ec0697c001e1e1779') } planSummary: IDHACK ntoskip:0 keysExamined:1 docsExamined:1 idhack:1 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:42 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 8015ms
2017-05-22T07:13:24.555+0000 I COMMAND [conn39] query databasename.collectionname query: { _id: ObjectId('576d44b8ea8351001ea5fb27') } planSummary: IDHACK ntoskip:0 keysExamined:1 docsExamined:1 idhack:1 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:42 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 8015ms
2017-05-22T07:13:24.561+0000 I COMMAND [conn34] query databasename.collectionname query: { _id: ObjectId('576d44b55de5fa001ebdd31e') } planSummary: IDHACK ntoskip:0 keysExamined:1 docsExamined:1 idhack:1 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:42 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 8022ms
2017-05-22T07:13:24.564+0000 I COMMAND [conn32] query databasename.collectionname query: { _id: ObjectId('576d4df6d738a7001ef2a235') } planSummary: IDHACK ntoskip:0 keysExamined:1 docsExamined:1 idhack:1 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:42 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 8025ms
2017-05-22T07:13:24.564+0000 I COMMAND [conn51] query databasename.collectionname query: { _id: ObjectId('576d48165de5fa001ebdd55a') } planSummary: IDHACK ntoskip:0 keysExamined:1 docsExamined:1 idhack:1 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:42 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 8024ms
2017-05-22T07:13:24.564+0000 I COMMAND [conn17] query databasename.collectionname query: { _id: ObjectId('576d44c19f2382001e953717') } planSummary: IDHACK ntoskip:0 keysExamined:1 docsExamined:1 idhack:1 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:42 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 8025ms
2017-05-22T07:13:24.564+0000 I COMMAND [conn8] query databasename.collectionname query: { _id: ObjectId('576d45d256c22e001efdb382') } planSummary: IDHACK ntoskip:0 keysExamined:1 docsExamined:1 idhack:1 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:42 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 8025ms
2017-05-22T07:13:24.564+0000 I COMMAND [conn42] query databasename.collectionname query: { _id: ObjectId('576d44bd57c75e001e6e2302') } planSummary: IDHACK ntoskip:0 keysExamined:1 docsExamined:1 idhack:1 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:42 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 8025ms
2017-05-22T07:13:24.564+0000 I COMMAND [conn6] query databasename.collectionname query: { _id: ObjectId('576d44b394e731001e7cd530') } planSummary: IDHACK ntoskip:0 keysExamined:1 docsExamined:1 idhack:1 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:42 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 8025ms
2017-05-22T07:13:24.571+0000 I COMMAND [conn31] query databasename.collectionname query: { _id: ObjectId('576d4dbcb7289f001e64e32b') } planSummary: IDHACK ntoskip:0 keysExamined:1 docsExamined:1 idhack:1 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:42 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 8032ms

This looks like very slow disk I/O. What does the planSummary: IDHACK mean? Any more info for IDHACK?

Answer 1

It is just informing that mongod has chosen to use the _id index.

More specifically: IDHACK means that normal query plan path (evaluation, caching, re-evaluation) was bypassed for the query shape {_id: <value>} which is supported by the required _id index. This is a performance optimization for the common use case of fetching a document by primary key.