In company we are working in project on very big database. It uses 100GB RAM. What's weird before first running a query PLE is 11k~, after running it drops to about 70, anyway when after 15 mins I check PLE again its about 1k~ and when I run query again it drops to 60. Why is it happening? If in time between running queries PLE increase doesn’t it mean that all needed data is in cache? If so why then running same query after 15 minutes cause PLE to drop again?

Here is the query:

```
select
    ResultType = case r.TypeID
        when 'dlp' then 'DLP'
        when 'bill' then 'BILL'
        when 'evtlog' then 'EVTLOG'
    end,
    SerialNumber, 
    ESerialNumber, 
    ResultDateTime, 
    DateTimeStamp as SavedInSystem
from 
    Results r
where
    TypeID in ('typeid1','typeid2')
    and DateTimeStamp > '2016-05-19 23:00:00'
    and SerialNumber in ('serialnumber')

```

I have a clustered index on `datetimestamp` and non clustered on `typeid, datetimestamp` and `resultdatetime, resultid, typeid` and few others...
@update

```
    select
(physical_memory_in_use_kb/1024)Memory_usedby_Sqlserver_MB,
(locked_page_allocations_kb/1024 )Locked_pages_used_Sqlserver_MB,
(total_virtual_address_space_kb/1024 )Total_VAS_in_MB,
process_physical_memory_low,
process_virtual_memory_low
from sys. dm_os_process_memory

```

returns 

```
Memory_usedby_Sqlserver_MB  Locked_pages_used_Sqlserver_MB  Total_VAS_in_MB process_physical_memory_low process_virtual_memory_low
102569  0   134217727   0   0

```

and here are counters, since freepages was removed in 2012 version i added few other counters

```
object_name counter_name instance_name cntr_value cntr_type
MSSQL$ServerNameC3SCW:Buffer Manager Database pages 11356975 65792
MSSQL$ServerNameC3SCW:Buffer Manager Checkpoint pages/sec 1053996662 272696576
MSSQL$ServerNameC3SCW:Buffer Manager Page life expectancy 4233 65792
MSSQL$ServerNameC3SCW:Buffer Node Database pages 003 2975519 65792
MSSQL$ServerNameC3SCW:Buffer Node Page life expectancy 003 4892 65792
MSSQL$ServerNameC3SCW:Buffer Node Database pages 002 2938151 65792
MSSQL$ServerNameC3SCW:Buffer Node Page life expectancy 002 4051 65792
MSSQL$ServerNameC3SCW:Buffer Node Database pages 001 3002872 65792
MSSQL$ServerNameC3SCW:Buffer Node Page life expectancy 001 4052 65792
MSSQL$ServerNameC3SCW:Buffer Node Database pages 000 2440433 65792
MSSQL$ServerNameC3SCW:Buffer Node Page life expectancy 000 4052 65792
MSSQL$ServerNameC3SCW:Memory Manager Database Cache Memory (KB) 90855800 65792
MSSQL$ServerNameC3SCW:Memory Manager Free Memory (KB) 286672 65792
MSSQL$ServerNameC3SCW:Memory Manager Memory Grants Pending 0 65792
MSSQL$ServerNameC3SCW:Memory Manager Total Server Memory (KB) 104857608 65792
MSSQL$ServerNameC3SCW:Memory Node Database Node Memory (KB) 003 23804152 65792
MSSQL$ServerNameC3SCW:Memory Node Free Node Memory (KB) 003 73256 65792
MSSQL$ServerNameC3SCW:Memory Node Database Node Memory (KB) 002 23505208 65792
MSSQL$ServerNameC3SCW:Memory Node Free Node Memory (KB) 002 76392 65792
MSSQL$ServerNameC3SCW:Memory Node Database Node Memory (KB) 001 24022976 65792
MSSQL$ServerNameC3SCW:Memory Node Free Node Memory (KB) 001 77504 65792
MSSQL$ServerNameC3SCW:Memory Node Database Node Memory (KB) 000 19523464 65792
MSSQL$ServerNameC3SCW:Memory Node Free Node Memory (KB) 000 59520 65792

```

and counters after query

```
object_name counter_name instance_name cntr_value cntr_type
MSSQL$ ServerNameC3SCW:Buffer Manager Database pages 11355652 65792
MSSQL$ ServerNameC3SCW:Buffer Manager Checkpoint pages/sec 1054000434 272696576
MSSQL$ ServerNameC3SCW:Buffer Manager Page life expectancy 310 65792
MSSQL$ ServerNameC3SCW:Buffer Node Database pages 003 2980418 65792
MSSQL$ ServerNameC3SCW:Buffer Node Page life expectancy 003 5417 65792
MSSQL$ ServerNameC3SCW:Buffer Node Database pages 002 2946298 65792
MSSQL$ ServerNameC3SCW:Buffer Node Page life expectancy 002 4591 65792
MSSQL$ ServerNameC3SCW:Buffer Node Database pages 001 2995850 65792
MSSQL$ ServerNameC3SCW:Buffer Node Page life expectancy 001 155 65792
MSSQL$ ServerNameC3SCW:Buffer Node Database pages 000 2433086 65792
MSSQL$ ServerNameC3SCW:Buffer Node Page life expectancy 000 165 65792
MSSQL$ ServerNameC3SCW:Memory Manager Database Cache Memory (KB) 90845216 65792
MSSQL$ ServerNameC3SCW:Memory Manager Free Memory (KB) 219240 65792
MSSQL$ ServerNameC3SCW:Memory Manager Memory Grants Pending 0 65792
MSSQL$ ServerNameC3SCW:Memory Manager Total Server Memory (KB) 104857608 65792
MSSQL$ ServerNameC3SCW:Memory Node Database Node Memory (KB) 003 23843344 65792
MSSQL$ ServerNameC3SCW:Memory Node Free Node Memory (KB) 003 51864 65792
MSSQL$ ServerNameC3SCW:Memory Node Database Node Memory (KB) 002 23570384 65792
MSSQL$ ServerNameC3SCW:Memory Node Free Node Memory (KB) 002 59680 65792
MSSQL$ ServerNameC3SCW:Memory Node Database Node Memory (KB) 001 23966800 65792
MSSQL$ ServerNameC3SCW:Memory Node Free Node Memory (KB) 001 58144 65792
MSSQL$ ServerNameC3SCW:Memory Node Database Node Memory (KB) 000 19464688 65792
MSSQL$ ServerNameC3SCW:Memory Node Free Node Memory (KB) 000 49552 65792
Target Server Memory (KB) MSSQL$GKKTSQLC3SCW:Memory Manager 104857608 65792

```

`SELECT @@version` returns:

```

Microsoft SQL Server 2012 - 11.0.5058.0 (X64) 
    May 14 2014 18:34:29 
    Copyright (c) Microsoft Corporation
    Enterprise Edition: Core-based Licensing (64-bit) on Windows NT 6.3  (Build 9600: )

```

In execution plan I can see that clustered index seek has cost 100%, sorting, hash matching has cost 0%. There is a hint about a missing index, but it would be an additional index over 5 columns for a table with 29 mln rows.