yugabyte/yugabyte-db

[YSQL] "Keyspace 'system_postgres' already exists" error when creating sequences concurrently

Open

#4,890 opened on Jun 26, 2020

View on GitHub
 (3 comments) (0 reactions) (0 assignees)C (1,003 forks)batch import
area/ysqlgood first issuekind/bugpriority/medium

Repository metrics

Stars
 (8,229 stars)
PR merge metrics
 (Avg merge 17d 21h) (92 merged PRs in 30d)

Description

Jira Link: DB-4818 We currently store some sequence data (last val, is_called, etc) in a dedicated DocDB system table, that exists in the system_postgres namespace. That namespace is not pre-created, so "create sequence" calls will create it if needed by calling CreateNamespaceIfNotExists.

However, CreateNamespaceIfNotExists does not correctly handle the "If Not Exists" aspect for YSQL. Specifically, it checks if the namespace exists before, but if it gets an IsAlreadyPresent status that still gets reported as an error. See the code in: https://github.com/yugabyte/yugabyte-db/blob/master/src/yb/client/client.cc#L654 -- there the IsAlreadyPresent check at the end only handles YQL_DATABASE_CQL namespaces. That means that if many sequences are created concurrently at least some (possibly all-but-one) will fail.

Contributor guide

Research direction
Examine the code at src/yb/client/client.cc around line 654 and src/yb/yql/pggate/pg session.cc around line 483. The CreateNamespaceIfNotExists function does not propagate the IsAlreadyPresent status for YSQL. To fix, add handling for YQL DATABASE PGSQL in the client.cc check, similar to the existing CQL handling.
Tech stack
ccpppostgresql
Domain
databasebackend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C++Distributed databases basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40