KDB specifies some default rules which are used to determine whether or not data will be compressed when sending that data over IPC.
Interprocess communication | Developing | kdb+ and q documentation - kdb+ and q documentation
Here is a copy of the relevant section from that documentation page.
Compression
For releases since 2012.05.29, kdb+ and the C-API will compress an outgoing message if
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Uncompressed serialized data has a length greater than 2000 bytes
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Connection is not
localhost -
Connection is not 127.0.0.1
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Connection is not using UDS (Unix Domain Socket)
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Connection does not resolve to being localhost (since 4.1t 2021.06.04)
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Size of compressed data is less than ½ the size of uncompressed data
Is it possible to disable compression for communication over IPC?