FAQ

The following document contains common hardware and software configuration issues met when running QuestDB, as well as solutions to them.

Why is ILP data not immediately available?#

InfluxDB line protocol (ILP) does not commit data on single lines or when the sender disconnects, but instead uses a number of rules to break incoming data into commit batches. This results in data not being visible in SELECT queries immediately after being received. Refer to InfluxDB line protocol guide to understand these rules.

How do I update or delete a row?#

See our guide on modifying data.

Why do I get table busy error messages when inserting data over PostgreSQL wire protocol?#

You may get table busy [reason=insert] or similar errors when running INSERT statements concurrently on the same table. This means that the table is locked by inserts issued from another SQL connection or other client protocols for data import, like ILP over TCP or CSV over HTTP. To reduce the chances of getting this error, try using auto-commit to keep the transaction as short as possible.

We're also considering adding automatic insert retries on the database side, but for now, it is safe to handle this error on the client side and retry the insert.

Why do I see could not open read-write messages when creating a table or inserting rows?#

Log messages may appear like the following:

2022-02-01T13:40:11.336011Z I i.q.c.l.t.LineTcpMeasurementScheduler could not create table [tableName=cpu, ex=could not open read-write
...
io.questdb.cairo.CairoException: [24] could not open read-only [file=/root/.questdb/db/cpu/service.k]

The machine may have insufficient limits for the maximum number of open files. Try checking the ulimit value on your machine. Refer to capacity planning page for more details.

Why do I see errno=12 mmap messages in the server logs?#

Log messages may appear like the following:

2022-02-01T13:40:10.636014Z E i.q.c.l.t.LineTcpConnectionContext [8655] could not process line data [table=test_table, msg=could not mmap [size=248, offset=0, fd=1766, memUsed=314809894008, fileLen=8192], errno=12]

The machine may have insufficient limits of memory map areas a process may have. Try checking and increasing the vm.max_map_count value on your machine. Refer to capacity planning page for more details.

How do I avoid duplicate rows with identical fields?#

We have an open feature request to optionally de-duplicate rows inserted with identical fields. Until then, you need to modify the data after it's inserted and use a GROUP BY query to identify duplicates.