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65	Introspection

Introspection Functions

You can use functions described in this chapter to introspect ELF and DWARF for query profiling.

!!! warning "Warning" These functions are slow and may impose security considerations.

For proper operation of introspection functions:

Install the clickhouse-common-static-dbg package.

Set the allow_introspection_functions setting to 1.

For security reasons introspection functions are disabled by default.

ClickHouse saves profiler reports to the trace_log system table. Make sure the table and profiler are configured properly.

addressToLine

Converts virtual memory address inside ClickHouse server process to the filename and the line number in ClickHouse source code.

If you use official ClickHouse packages, you need to install the clickhouse-common-static-dbg package.

Syntax

addressToLine(address_of_binary_instruction)

Arguments

address_of_binary_instruction (UInt64) — Address of instruction in a running process.

Returned value

Source code filename and the line number in this file delimited by colon.

For example, `/build/obj-x86_64-linux-gnu/../src/Common/ThreadPool.cpp:199`, where `199` is a line number.

Name of a binary, if the function couldn’t find the debug information.
Empty string, if the address is not valid.

Type: String.

Example

Enabling introspection functions:

SET allow_introspection_functions=1

Selecting the first string from the trace_log system table:

SELECT * FROM system.trace_log LIMIT 1 \G

Row 1:
──────
event_date:              2019-11-19
event_time:              2019-11-19 18:57:23
revision:                54429
timer_type:              Real
thread_number:           48
query_id:                421b6855-1858-45a5-8f37-f383409d6d72
trace:                   [140658411141617,94784174532828,94784076370703,94784076372094,94784076361020,94784175007680,140658411116251,140658403895439]

The trace field contains the stack trace at the moment of sampling.

Getting the source code filename and the line number for a single address:

SELECT addressToLine(94784076370703) \G

Row 1:
──────
addressToLine(94784076370703): /build/obj-x86_64-linux-gnu/../src/Common/ThreadPool.cpp:199

Applying the function to the whole stack trace:

SELECT
    arrayStringConcat(arrayMap(x -> addressToLine(x), trace), '\n') AS trace_source_code_lines
FROM system.trace_log
LIMIT 1
\G

The arrayMap function allows to process each individual element of the trace array by the addressToLine function. The result of this processing you see in the trace_source_code_lines column of output.

Row 1:
──────
trace_source_code_lines: /lib/x86_64-linux-gnu/libpthread-2.27.so
/usr/lib/debug/usr/bin/clickhouse
/build/obj-x86_64-linux-gnu/../src/Common/ThreadPool.cpp:199
/build/obj-x86_64-linux-gnu/../src/Common/ThreadPool.h:155
/usr/include/c++/9/bits/atomic_base.h:551
/usr/lib/debug/usr/bin/clickhouse
/lib/x86_64-linux-gnu/libpthread-2.27.so
/build/glibc-OTsEL5/glibc-2.27/misc/../sysdeps/unix/sysv/linux/x86_64/clone.S:97

addressToSymbol

Converts virtual memory address inside ClickHouse server process to the symbol from ClickHouse object files.

Syntax

addressToSymbol(address_of_binary_instruction)

Arguments

address_of_binary_instruction (UInt64) — Address of instruction in a running process.

Returned value

Symbol from ClickHouse object files.
Empty string, if the address is not valid.

Type: String.

Example

Enabling introspection functions:

SET allow_introspection_functions=1

Selecting the first string from the trace_log system table:

SELECT * FROM system.trace_log LIMIT 1 \G

Row 1:
──────
event_date:    2019-11-20
event_time:    2019-11-20 16:57:59
revision:      54429
timer_type:    Real
thread_number: 48
query_id:      724028bf-f550-45aa-910d-2af6212b94ac
trace:         [94138803686098,94138815010911,94138815096522,94138815101224,94138815102091,94138814222988,94138806823642,94138814457211,94138806823642,94138814457211,94138806823642,94138806795179,94138806796144,94138753770094,94138753771646,94138753760572,94138852407232,140399185266395,140399178045583]

The trace field contains the stack trace at the moment of sampling.

Getting a symbol for a single address:

SELECT addressToSymbol(94138803686098) \G

Row 1:
──────
addressToSymbol(94138803686098): _ZNK2DB24IAggregateFunctionHelperINS_20AggregateFunctionSumImmNS_24AggregateFunctionSumDataImEEEEE19addBatchSinglePlaceEmPcPPKNS_7IColumnEPNS_5ArenaE

Applying the function to the whole stack trace:

SELECT
    arrayStringConcat(arrayMap(x -> addressToSymbol(x), trace), '\n') AS trace_symbols
FROM system.trace_log
LIMIT 1
\G

The arrayMap function allows to process each individual element of the trace array by the addressToSymbols function. The result of this processing you see in the trace_symbols column of output.

Row 1:
──────
trace_symbols: _ZNK2DB24IAggregateFunctionHelperINS_20AggregateFunctionSumImmNS_24AggregateFunctionSumDataImEEEEE19addBatchSinglePlaceEmPcPPKNS_7IColumnEPNS_5ArenaE
_ZNK2DB10Aggregator21executeWithoutKeyImplERPcmPNS0_28AggregateFunctionInstructionEPNS_5ArenaE
_ZN2DB10Aggregator14executeOnBlockESt6vectorIN3COWINS_7IColumnEE13immutable_ptrIS3_EESaIS6_EEmRNS_22AggregatedDataVariantsERS1_IPKS3_SaISC_EERS1_ISE_SaISE_EERb
_ZN2DB10Aggregator14executeOnBlockERKNS_5BlockERNS_22AggregatedDataVariantsERSt6vectorIPKNS_7IColumnESaIS9_EERS6_ISB_SaISB_EERb
_ZN2DB10Aggregator7executeERKSt10shared_ptrINS_17IBlockInputStreamEERNS_22AggregatedDataVariantsE
_ZN2DB27AggregatingBlockInputStream8readImplEv
_ZN2DB17IBlockInputStream4readEv
_ZN2DB26ExpressionBlockInputStream8readImplEv
_ZN2DB17IBlockInputStream4readEv
_ZN2DB26ExpressionBlockInputStream8readImplEv
_ZN2DB17IBlockInputStream4readEv
_ZN2DB28AsynchronousBlockInputStream9calculateEv
_ZNSt17_Function_handlerIFvvEZN2DB28AsynchronousBlockInputStream4nextEvEUlvE_E9_M_invokeERKSt9_Any_data
_ZN14ThreadPoolImplI20ThreadFromGlobalPoolE6workerESt14_List_iteratorIS0_E
_ZZN20ThreadFromGlobalPoolC4IZN14ThreadPoolImplIS_E12scheduleImplIvEET_St8functionIFvvEEiSt8optionalImEEUlvE1_JEEEOS4_DpOT0_ENKUlvE_clEv
_ZN14ThreadPoolImplISt6threadE6workerESt14_List_iteratorIS0_E
execute_native_thread_routine
start_thread
clone

demangle

Converts a symbol that you can get using the addressToSymbol function to the C++ function name.

Syntax

demangle(symbol)

Arguments

symbol (String) — Symbol from an object file.

Returned value

Name of the C++ function.
Empty string if a symbol is not valid.

Type: String.

Example

Enabling introspection functions:

SET allow_introspection_functions=1

Selecting the first string from the trace_log system table:

SELECT * FROM system.trace_log LIMIT 1 \G

Row 1:
──────
event_date:    2019-11-20
event_time:    2019-11-20 16:57:59
revision:      54429
timer_type:    Real
thread_number: 48
query_id:      724028bf-f550-45aa-910d-2af6212b94ac
trace:         [94138803686098,94138815010911,94138815096522,94138815101224,94138815102091,94138814222988,94138806823642,94138814457211,94138806823642,94138814457211,94138806823642,94138806795179,94138806796144,94138753770094,94138753771646,94138753760572,94138852407232,140399185266395,140399178045583]

The trace field contains the stack trace at the moment of sampling.

Getting a function name for a single address:

SELECT demangle(addressToSymbol(94138803686098)) \G

Row 1:
──────
demangle(addressToSymbol(94138803686098)): DB::IAggregateFunctionHelper<DB::AggregateFunctionSum<unsigned long, unsigned long, DB::AggregateFunctionSumData<unsigned long> > >::addBatchSinglePlace(unsigned long, char*, DB::IColumn const**, DB::Arena*) const

Applying the function to the whole stack trace:

SELECT
    arrayStringConcat(arrayMap(x -> demangle(addressToSymbol(x)), trace), '\n') AS trace_functions
FROM system.trace_log
LIMIT 1
\G

The arrayMap function allows to process each individual element of the trace array by the demangle function. The result of this processing you see in the trace_functions column of output.

Row 1:
──────
trace_functions: DB::IAggregateFunctionHelper<DB::AggregateFunctionSum<unsigned long, unsigned long, DB::AggregateFunctionSumData<unsigned long> > >::addBatchSinglePlace(unsigned long, char*, DB::IColumn const**, DB::Arena*) const
DB::Aggregator::executeWithoutKeyImpl(char*&, unsigned long, DB::Aggregator::AggregateFunctionInstruction*, DB::Arena*) const
DB::Aggregator::executeOnBlock(std::vector<COW<DB::IColumn>::immutable_ptr<DB::IColumn>, std::allocator<COW<DB::IColumn>::immutable_ptr<DB::IColumn> > >, unsigned long, DB::AggregatedDataVariants&, std::vector<DB::IColumn const*, std::allocator<DB::IColumn const*> >&, std::vector<std::vector<DB::IColumn const*, std::allocator<DB::IColumn const*> >, std::allocator<std::vector<DB::IColumn const*, std::allocator<DB::IColumn const*> > > >&, bool&)
DB::Aggregator::executeOnBlock(DB::Block const&, DB::AggregatedDataVariants&, std::vector<DB::IColumn const*, std::allocator<DB::IColumn const*> >&, std::vector<std::vector<DB::IColumn const*, std::allocator<DB::IColumn const*> >, std::allocator<std::vector<DB::IColumn const*, std::allocator<DB::IColumn const*> > > >&, bool&)
DB::Aggregator::execute(std::shared_ptr<DB::IBlockInputStream> const&, DB::AggregatedDataVariants&)
DB::AggregatingBlockInputStream::readImpl()
DB::IBlockInputStream::read()
DB::ExpressionBlockInputStream::readImpl()
DB::IBlockInputStream::read()
DB::ExpressionBlockInputStream::readImpl()
DB::IBlockInputStream::read()
DB::AsynchronousBlockInputStream::calculate()
std::_Function_handler<void (), DB::AsynchronousBlockInputStream::next()::{lambda()#1}>::_M_invoke(std::_Any_data const&)
ThreadPoolImpl<ThreadFromGlobalPool>::worker(std::_List_iterator<ThreadFromGlobalPool>)
ThreadFromGlobalPool::ThreadFromGlobalPool<ThreadPoolImpl<ThreadFromGlobalPool>::scheduleImpl<void>(std::function<void ()>, int, std::optional<unsigned long>)::{lambda()#3}>(ThreadPoolImpl<ThreadFromGlobalPool>::scheduleImpl<void>(std::function<void ()>, int, std::optional<unsigned long>)::{lambda()#3}&&)::{lambda()#1}::operator()() const
ThreadPoolImpl<std::thread>::worker(std::_List_iterator<std::thread>)
execute_native_thread_routine
start_thread
clone

tid

Returns id of the thread, in which current Block is processed.

Syntax

tid()

Returned value

Current thread id. Uint64.

Example

Query:

SELECT tid();

Result:

┌─tid()─┐
│  3878 │
└───────┘

logTrace

Emits trace log message to server log for each Block.

Syntax

logTrace('message')

Arguments

message — Message that is emitted to server log. String.

Returned value

Always returns 0.

Example

Query:

SELECT logTrace('logTrace message');

Result:

┌─logTrace('logTrace message')─┐
│                            0 │
└──────────────────────────────┘

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Introspection Functions

addressToLine

addressToSymbol

demangle

tid

logTrace

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