Agent Skill · Cockroach Labs

molt-fetch

Guide for using molt fetch to migrate data from PostgreSQL, MySQL, Oracle, or MSSQL to CockroachDB. Use when running molt fetch commands, configuring storage backends, handling fetch failures/resumption, or chaining fetch with verify.

View SKILL.md on GitHub → Source repository Provider profile
Provider: Cockroach Labs Path in repo: skills/cockroachdb-onboarding-and-migrations/molt-fetch/SKILL.md

Skill body

molt fetch

Bulk data migration from source databases (PostgreSQL, MySQL, Oracle, MSSQL) to CockroachDB.

Basic Structure

molt fetch \
  --source "<source-conn>" \
  --target "<crdb-conn>" \
  --bucket-path "s3://bucket/prefix"   # or --direct-copy or --local-path
  [options]

Storage Backends (pick one)

Option When to use
--bucket-path "s3://..." AWS S3 (also gs:// for GCS, azure:// for Azure)
--direct-copy No intermediate storage; fastest for accessible networks
--local-path "/tmp/molt" + --local-path-listen-addr "0.0.0.0:9005" CRDB must reach the listen addr

Cloud auth: pass --use-implicit-auth for IAM/ADC/managed identity, or set AWS_ACCESS_KEY_ID/GOOGLE_APPLICATION_CREDENTIALS env vars.

Table Handling (--table-handling)

Value Behavior
none (default) Append to existing tables
drop-on-target-and-recreate Drop + recreate from source schema; enables auto schema creation
truncate-if-exists Truncate before loading; errors if table missing

Import Mode

IMPORT INTO (default): Table goes OFFLINE during load. Highest throughput.

COPY FROM (--use-copy): Table stays ONLINE. Use with --direct-copy. Cannot use compression.

# Zero-downtime load
molt fetch --source "..." --target "..." --direct-copy --use-copy

Key Flags

# Filtering
--table-filter "customers|orders"      # POSIX regex for tables to include
--table-exclusion-filter "temp_.*"     # exclude pattern
--schema-filter "public"               # PostgreSQL only

# Performance
--table-concurrency 4                  # parallel tables (default: 4)
--export-concurrency 4                 # export threads (default: 4)
--row-batch-size 100000                # rows per SELECT (default: 100k)

# Schema
--type-map-file "types.json"           # custom type mappings
--transformations-file "transforms.json"  # column exclusions, table aliases

# Logging
--log-file "migration.log"             # or "stdout"
--logging debug                        # info (default), debug, trace
--metrics-listen-addr "0.0.0.0:3030"  # Prometheus scrape endpoint

Source-Specific Prerequisites

MySQL: GTID mode required (gtid_mode=ON, enforce_gtid_consistency=ON). ONLY_FULL_GROUP_BY must be off. Or use --ignore-replication-check.

Oracle: Binary must be built with CGO_ENABLED=1 -tags="cgo source_all". Oracle Instant Client in LD_LIBRARY_PATH.

PostgreSQL: Replication privileges needed, or --ignore-replication-check.

Common Workflows

1. Full migration with schema creation

molt fetch \
  --source "postgresql://user:pass@pg:5432/db" \
  --target "postgresql://root@crdb:26257/db" \
  --bucket-path "s3://mybucket/migration" \
  --table-handling drop-on-target-and-recreate \
  --table-filter "customers|orders|payments" \
  --log-file migration.log

2. Resume after failure

# List available continuation tokens
molt fetch tokens --fetch-id "abc-123" --target "postgresql://root@crdb:26257/db"

# Resume all failed tables
molt fetch \
  --source "..." --target "..." \
  --bucket-path "s3://mybucket/migration" \
  --fetch-id "abc-123" \
  --non-interactive

Error Recovery

Error Cause Fix
“GTID-based replication not enabled” MySQL missing GTID Enable gtid_mode=ON or add --ignore-replication-check
“Column mismatch” Schema diverged Fix target schema manually or use --type-map-file
Silent IMPORT INTO CockroachDB import running SHOW JOBS on CRDB to check progress
“timestamp in the future” Docker/Mac clock drift Sync clocks between hosts

Gotchas

See flags reference for the full flag list.

Skill frontmatter

compatibility: Requires molt binary. Source DB must be accessible. For Oracle, CGO and Oracle Instant Client required. metadata: {"author" => "cockroachdb", "version" => "1.0"}

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