query_system_catalog — SELECT/SHOW/EXPLAIN restricted to system catalog tables (mz_*, pg_catalog, information_schema). Takes no cluster argument, and one passed anyway is silently ignored rather than rejected. Catalog reads are auto-routed to the catalog server cluster (mz_catalog_server) while auto_route_catalog_queries is on (the default); anything the router cannot serve there — notably every mz_introspection relation — runs on the session's default cluster (the role's cluster default if set, else the system default, quickstart unless the operator changed it). Use for most catalog lookups.
query (added in Materialize v26.30) — SELECT/SHOW/EXPLAIN against any object the role can access, on the cluster named by its required cluster argument (plus, from v26.33, an optional cluster_replica). Required for EXPLAIN ANALYZE (it must run on the MV/index's cluster) and for reading user data directly. May be hidden when the operator has disabled enable_mcp_developer_query_tool.

Connecting an MCP client to `materialize-developer`

If the user is asking how to configure, connect, or set up an MCP client (Claude Code, Cursor, VS Code, Zed, Continue, Windsurf, Claude Desktop) to talk to the materialize-developer server — including how to control which user or role the connection uses, or how to switch between identities — see mcp-client-connect.md. It covers the Emulator, Materialize Cloud, and self-managed deployments, with per-client snippets, authentication patterns, and verification recipes.

The rest of this SKILL.md covers what to do once you are connected and want to analyze the environment.

Discovering Tables and Columns

Do NOT guess column names. System catalog relations are a mix of tables, views, materialized views and sources, so no single SHOW TABLES or SHOW VIEWS lists them all; use SHOW OBJECTS FROM <schema>. The ontology views in mz_internal describe the catalog; use them to discover the correct tables, columns, join paths, and ID types:

Table	What it tells you
`mz_internal.mz_ontology_entity_types`	What catalog entities exist and which `mz_*` table they map to.
`mz_internal.mz_ontology_link_types`	Relationships between entities (foreign keys, metrics, etc.).
`mz_internal.mz_ontology_properties`	Column names, types, and descriptions for each entity.
`mz_internal.mz_ontology_semantic_types`	Typed ID domains (CatalogItemId, ReplicaId, etc.).

Example queries:

-- Find the right table for an entity
SELECT name, relation, description
FROM mz_internal.mz_ontology_entity_types
WHERE name LIKE '%source%'

-- Find join paths between entities
SELECT name, source_entity, target_entity, properties, description
FROM mz_internal.mz_ontology_link_types
WHERE source_entity = 'source' OR target_entity = 'source'

-- Find columns for a table
SELECT column_name, semantic_type, description
FROM mz_internal.mz_ontology_properties
WHERE entity_type = 'source_status'

Verify column names with SHOW COLUMNS FROM <schema>.<table> before querying.

Critical Rules

The server's own initialize instructions carry the catalog gotchas: column names such as last_status_change_at, the replica-to-cluster JOIN for mz_cluster_replica_utilization, the ontology views, and from v26.40 the mz_introspection routing and id types. Most clients forward them; if yours does not, SHOW COLUMNS is the fallback.

`mz_introspection` relations need the `query` tool

Every mz_introspection relation, mz_dataflow_arrangement_sizes included, is cluster-scoped, and query_system_catalog cannot target a cluster, so through that tool the read lands on the session's default cluster: with exactly one replica there you get its numbers, or an empty result, with no error; with several replicas the read fails with log source reads must target a replica, with none with has no replicas available to service request. Read them through query with the cluster argument, plus cluster_replica (from v26.33) on a cluster with more than one replica (unbilled ones count), which is what the first error asks for; nothing fixes the second. Servers before v26.40 say in their initialize instructions never to query mz_dataflow_arrangement_sizes; that rule predates the query tool, and this supersedes it. The query, its join path to the catalog, and its caveats are under Arrangement Sizes in references/queries.md. Without the query tool, fall back to the relations under Memory and Resource Consumption below.

Workflow Overview

Connect — Verify the MCP Developer tools are available
Discover — Use the ontology + catalog queries to inventory all deployed objects
Analyze — Assess performance metrics: freshness, hydration, memory, utilization
Report — Produce a structured markdown report with findings and recommendations

Step 1: Verify MCP Connection

Confirm you have access to the query_system_catalog tool. Run a quick test:

query_system_catalog: SELECT mz_version() FROM mz_catalog.mz_databases LIMIT 1

query_system_catalog rejects a SELECT that references no system catalog table, so a bare SELECT mz_version() fails with Query must reference at least one system catalog table, which reads like a broken connection.

Check whether a query tool is among the tools your client exposes. With it you can also run EXPLAIN ANALYZE and queries against user objects on a named cluster; without it (operator disabled it, or a pre-v26.30 build), fall back to query_system_catalog for everything that fits.

If query_system_catalog fails, check:

The MCP server is registered in your client's MCP configuration (the file and key differ per client, see mcp-client-connect.md)
The enable_mcp_developer feature flag is enabled on the environment
Your authentication credentials are valid

Running Queries

Both tools take one read-only statement per call, no SET, and no trailing semicolon (one is tolerated; two statements are rejected).

Results come back as a bare array of rows with no column names, and every AS alias is discarded, so map columns positionally and keep the SELECT list short and in the order you intend to read it. Numbers arrive as strings, booleans as JSON true/false, NULL as null; timestamp and timestamptz values arrive as millisecond-since-epoch strings (1787687415336.000), so cast them to text where you need a readable time. Qualify object names with the database when the object is not in the session's database (SHOW database), or the statement fails with unknown schema. A response is capped (1 MB by default) and a request timed out (60 seconds by default), both configurable by the operator and both surfacing as errors. A LIMIT keeps a catalog enumeration under the cap, and shortens the work only for a plain read of one indexed relation, not for a join. A timeout releases the call but sometimes not the query, so do not retry a timed-out statement as it was.

query_system_catalog checks its allowlist by walking FROM-clause table references, so statements without one are not checked at all: SHOW TABLES, SHOW COLUMNS, the SHOW CREATE forms and object-level EXPLAIN ANALYZE pass there and can name user objects (which is why the latter answers empty, see Worker Skew). Treat that as a fallback for when query is missing; use query for user objects.

When filtering out system schemas, always exclude: mz_catalog, mz_internal, pg_catalog, information_schema, and mz_introspection. SHOW SCHEMAS also lists mz_catalog_unstable and mz_unsafe, which hold no relations.

Step 2: Discover — Inventory the Environment

Run the discovery queries to understand what is deployed. See references/queries.md for the full query set. The discovery phase covers:

Environment Overview

Materialize version
Clusters and replicas — names, managed, replica counts and sizes. Start from mz_clusters and LEFT JOIN the replicas, or a cluster without replicas disappears from the inventory. An unmanaged cluster has NULL size and replication_factor; unbilled support replicas are listed in mz_internal.mz_internal_cluster_replicas and count like any other.
Schemas in use

Deployed Objects Inventory

Sources: type (Kafka, Postgres, MySQL, Webhook, etc.), cluster assignment, status. mz_sources also has a row for every subsource and progress collection, so filter type NOT IN ('subsource', 'progress') before reporting a source count.
Materialized Views: cluster assignment, indexes, dependencies
Views: (non-materialized) and their usage patterns
Sinks: type, destination, cluster assignment
Indexes: what they're on, cluster assignment
Connections: external system connections configured

Object Definitions

Retrieve SQL definitions for materialized views, views, indexes, and sources using references/queries.md. This is critical for optimization analysis — the SQL definitions tell you how things are computed:

Join patterns and join order
Filter predicates (or lack thereof — missing temporal filters are a common issue)
Aggregation strategies
Whether MVs duplicate logic that could be shared

Step 3: Analyze — Performance and Resource Metrics

Freshness (Lag Analysis)

Query mz_internal.mz_materialization_lag for per-object lag. Its local_lag and global_lag columns are plain intervals, so they compare and sort directly, and slowest_global_input_id names the input holding the object back. Both lags are measured against the inputs' frontiers, not the wall clock: a pipeline whose source is paused shows zero lag while growing arbitrarily stale, so also compare mz_frontiers.write_frontier with now() (the Write Frontiers query).

write_frontier is not on this relation. It lives on mz_internal.mz_frontiers as an mz_timestamp, which casts to timestamptz for a readable time and to text (then bigint) for the raw number, never directly to bigint. references/queries.md has the queries.

Hydration Status

Query mz_internal.mz_hydration_statuses to check whether all dataflows are hydrated. Non-hydrated objects after initial startup may indicate resource pressure or configuration issues. LEFT JOIN the replica columns: replica_id is NULL with no replica to hydrate on, and an inner join hides those objects.

Paused Clusters Holding Back Compaction

A materialized view on a user cluster with no replicas holds back compaction of every input it reads, so their storage keeps growing, and Materialize raises no notice about it. Check for it in every health review and put any hit in the executive summary. Run the Materialized Views on Clusters with No Replicas query in references/queries.md. To fix it, give the cluster a replica, which releases the inputs right away, or drop the view if nobody needs it. A managed cluster takes ALTER CLUSTER <name> SET (REPLICATION FACTOR = 1), an unmanaged one CREATE CLUSTER REPLICA <name>.<replica> (SIZE = '<size>'). Indexes on a cluster with no replicas and SCHEDULE = ON REFRESH clusters don't hold back their inputs.

Memory and Resource Consumption

mz_internal.mz_cluster_replica_utilization for memory/CPU percentage per replica process
mz_internal.mz_cluster_replica_metrics for raw memory metrics, also one row per (replica, process)
mz_internal.mz_index_advice to identify which MVs/indexes can be optimized

Worker Skew (CPU imbalance across workers)

Use WITH SKEW to find operators where one worker does disproportionate CPU/memory work.

Run these through the query tool, not query_system_catalog: EXPLAIN ANALYZE executes on the cluster you pass as the cluster argument, and for the object-level commands that must be the cluster the MV/index lives on. Getting it wrong does not reliably error: on another single-replica cluster, or through query_system_catalog when the default cluster has one replica, the object-level commands return an empty result, which reads like "no skew"; with several replicas the read fails with log source reads must target a replica, with none with has no replicas available to service request. Without query, this section is not actionable.

Cluster-level (run on the cluster you want to inspect):

EXPLAIN ANALYZE CLUSTER CPU WITH SKEW

Object-level (run for both the MV and its indexes):

EXPLAIN ANALYZE CPU WITH SKEW FOR MATERIALIZED VIEW <schema>.<mv_name>

EXPLAIN ANALYZE CPU WITH SKEW FOR INDEX <schema>.<index_name>

If skew is present: identify the skewed operator (often TopK/window/agg/join/distinct), then inspect definitions (SHOW CREATE ...) and recommend a concrete SQL change (remove/adjust hints, refactor keys/partitioning, or rewrite the MV).

Index Advice

Query mz_internal.mz_index_advice, Materialize's built-in advisor. It emits exactly six hints: keep, drop unless queried directly, convert to a view, convert to a view with an index, convert to materialized view, add index. Filtering on a subset silently drops real recommendations; references/queries.md says what each one means.

Cost Analysis (optional)

Use the Cost Analysis queries in references/queries.md (per-replica pricing).

When writing recommendations, always quantify the credit impact (credits are a Cloud billing unit; on self-managed the shipped sizes carry Cloud's numbers and operator-defined sizes whatever the operator set, 0 by default, so present them as relative weights at best).

Object Dependencies

Query mz_internal.mz_object_dependencies to understand the dependency graph.

Step 4: Report — Generate the Analysis

Produce a structured markdown report following references/report-template.md: executive summary, cluster topology, deployed objects, performance analysis (freshness, hydration, utilization, worker skew, source and sink health), cost analysis, index advice, SQL-level analysis, and numbered recommendations with specific SQL.

The Cluster Topology table comes from the Cluster Topology query in references/queries.md plus the credit columns of Current Compute Cost per Cluster there.

Writing Recommendations

Always include specific SQL commands. For example:

Good:

Recommendation: Dematerialize my_schema.unused_mv to save memory.

SHOW CREATE MATERIALIZED VIEW my_schema.unused_mv
DROP MATERIALIZED VIEW my_schema.unused_mv
CREATE VIEW my_schema.unused_mv AS <definition>

Bad:

Recommendation: Consider dematerializing my_schema.unused_mv.

Troubleshooting Runbooks

Always end with specific SQL commands to fix the issue, and never apply one without the user's yes: scaling up or adding replicas costs money, and dropping or altering objects loses state. Every fix below is DDL, which both tools reject, so hand the commands to the user.

"Why is my materialized view stale?"

Diagnostic steps:

Check whether the MV's cluster has any replica first: mz_clusters LEFT JOIN mz_cluster_replicas (the Clusters and Replicas query), not replication_factor, which is NULL on unmanaged clusters and blind to unbilled replicas. A cluster with no replicas runs nothing, and the steps below don't work well.
Check mz_internal.mz_hydration_statuses — is the MV hydrated, and is everything else on its cluster (the Non-Hydrated Objects query, restricted to that cluster)?
Check mz_internal.mz_cluster_replica_status_history for recent offline events with reason oom-killed, which covers the cgroup OOM killer, the heap limiter and a full lgalloc spill disk alike (the Replica Restarts query; the emulator records no reason, so there judge by the offline count): an OOM loop often reads online in the current status and modest in a utilization sample. Most OOMs happen during hydration, so the loop is over when step 2 shows everything hydrated and the latest history row per process is online; kills that recur after hydration completes are a steady-state loop, which only a size change or less on the cluster ends.
Check mz_internal.mz_cluster_replica_utilization: memory_percent near 100 means RAM is full and further growth lands on swap where there is swap, felt as slow hydration or lag; heap_percent (RAM plus swap against the limit) climbing while memory_percent stays put is that swap in use, and near 100 the next spike kills the replica. Either way: scale up, move the MV, or shrink what the cluster holds (the fixes below, and step 7).
Check mz_internal.mz_source_statuses — upstream source errors?
Check mz_internal.mz_materialization_lag for the MV's lag (see Freshness)
If the query tool is available, run EXPLAIN ANALYZE MEMORY FOR MATERIALIZED VIEW <schema>.<mv> on the MV's cluster to see per-operator memory and spot expensive shapes (large arrangements, joins without indexes, missing temporal filters).

Common fixes:

If the MV's cluster has no replicas:

ALTER CLUSTER <cluster_name> SET (REPLICATION FACTOR 1)

-- Unmanaged cluster: add a replica directly
CREATE CLUSTER REPLICA <cluster_name>.r1 (SIZE = '<size>')

If the cluster is overloaded (high memory/CPU):

-- Option A: Scale up the cluster (unmanaged: DROP and CREATE its replicas
-- at the bigger size instead)
ALTER CLUSTER <cluster_name> SET (SIZE = '<next_size_up>')

-- Option B: Move the MV to a different cluster
SHOW CREATE MATERIALIZED VIEW <schema>.<mv_name>
DROP MATERIALIZED VIEW <schema>.<mv_name>
CREATE MATERIALIZED VIEW <schema>.<mv_name> IN CLUSTER <new_cluster> AS <definition>

If an upstream source is not running:

SELECT name, status, error, last_status_change_at
FROM mz_internal.mz_source_statuses
WHERE status != 'running'

Not every non-running status is a fault, and only stalled carries an error; "Are my sources healthy?" below lists what each status means. Fix the upstream source issue first. MV freshness depends on source health.

"Why is my cluster running out of memory?"

Diagnostic steps:

Check mz_internal.mz_cluster_replica_utilization for memory percentage and mz_cluster_replica_status_history for oom-killed events (Replica Restarts)
Check mz_internal.mz_index_advice for MVs that can be dematerialized
Check MV definitions for missing temporal filters
Check for redundant indexes
If the query tool is available, run EXPLAIN ANALYZE MEMORY FOR MATERIALIZED VIEW <schema>.<mv> (or FOR INDEX <name>) on the suspect object's cluster to see which operators hold the most memory — the most direct way to confirm which arrangement is responsible.

Common fixes:

Dematerialize MVs that don't need to be materialized:

SELECT o.name, o.type, sc.name AS schema_name, ia.hint, ia.details
FROM mz_internal.mz_index_advice ia
JOIN mz_catalog.mz_objects o ON ia.object_id = o.id
JOIN mz_catalog.mz_schemas sc ON o.schema_id = sc.id
WHERE ia.hint = 'convert to a view'

convert to a view with an index and convert to materialized view are memory recommendations too, with different remediations, so read the whole advice set (the Index Advice query in references/queries.md) before acting on this one.

-- For each candidate:
SHOW CREATE MATERIALIZED VIEW <schema>.<mv_name>
DROP MATERIALIZED VIEW <schema>.<mv_name>
CREATE VIEW <schema>.<mv_name> AS <definition>

Drop unused indexes:

SELECT o.name, o.type, sc.name AS schema_name, ia.hint, ia.details
FROM mz_internal.mz_index_advice ia
JOIN mz_catalog.mz_objects o ON ia.object_id = o.id
JOIN mz_catalog.mz_schemas sc ON o.schema_id = sc.id
WHERE ia.hint = 'drop unless queried directly'

-- Verify with the user before dropping
DROP INDEX <schema>.<index_name>

Scale up the cluster (unmanaged: DROP and CREATE its replicas at the bigger size instead):

ALTER CLUSTER <cluster_name> SET (SIZE = '<next_size_up>')

"Are my sources healthy? / Has my source finished snapshotting?"

Diagnostic steps:

Check mz_internal.mz_source_statuses for source errors
Check mz_internal.mz_source_statistics for ingestion progress
Check mz_internal.mz_materialization_lag for end-to-end lag

Common fixes:

If a source is not running:

SELECT name, status, error
FROM mz_internal.mz_source_statuses
WHERE status != 'running'

The status is one of created (no status recorded yet), starting, running, paused, stalled or dropped; the docs also list failed, which the code never produces. Only stalled carries an error. Webhook and progress rows always read running. created and starting are transient and normal; paused means the source's cluster has no replicas, which no amount of waiting fixes:

-- paused: give the source's cluster a replica
ALTER CLUSTER <cluster_name> SET (REPLICATION FACTOR 1)
-- unmanaged cluster: CREATE CLUSTER REPLICA <cluster_name>.r1 (SIZE = '<size>')

-- stalled/failed on bad credentials:
ALTER SECRET <secret_name> AS '<new_value>'

If snapshot_committed is false, the source is still loading its initial snapshot. This is normal for large sources — wait for it to complete; it also reads false with zero counters for about two minutes after a source or replica starts. A source with no mz_source_statistics row at all never ran, so check its status and whether its cluster has any replica instead of waiting; a paused source keeps a stale row, so read status first.

"What's the health of my environment?"

Run these checks in order:

mz_internal.mz_cluster_replica_statuses — all replicas of user clusters ready, and no recent oom-killed in mz_cluster_replica_status_history?
Paused clusters — any materialized view on a user cluster with no replicas? It holds back compaction of its inputs (Paused Clusters Holding Back Compaction).
mz_internal.mz_source_statuses — all sources running?
mz_internal.mz_sink_statuses — all sinks running?
mz_internal.mz_cluster_replica_utilization — resource pressure?

"What can I optimize to save costs?"

Check mz_internal.mz_index_advice for optimization candidates
Check cluster utilization — are clusters over-provisioned?
Check credit rates from mz_catalog.mz_cluster_replica_sizes

Notes

Access is governed by RBAC, so you only see the data your credentials have access to. The catalog is not gated the same way: a role with no object grants still reads the full object inventory, every view and MV definition, and all of mz_index_advice. Where RBAC is on, query and every read that is not auto-routed also need USAGE on the cluster they run on (mz_catalog_server grants it to PUBLIC). Whether RBAC checks are on depends on the deployment and its configuration; SHOW enable_rbac_checks and SHOW enable_session_rbac_checks tell (either on enables them).
Freshness numbers are point-in-time snapshots. Re-run to check if lag is stable or growing.

Connecting an MCP client to materialize-developer

Discovering Tables and Columns

Critical Rules

mz_introspection relations need the query tool

Workflow Overview

Step 1: Verify MCP Connection

Running Queries

Step 2: Discover — Inventory the Environment

Environment Overview

Deployed Objects Inventory

Object Definitions

Step 3: Analyze — Performance and Resource Metrics

Freshness (Lag Analysis)

Hydration Status

Paused Clusters Holding Back Compaction

Memory and Resource Consumption

Worker Skew (CPU imbalance across workers)

Index Advice

Cost Analysis (optional)

Object Dependencies

Step 4: Report — Generate the Analysis

Writing Recommendations

Troubleshooting Runbooks

"Why is my materialized view stale?"

"Why is my cluster running out of memory?"

"Are my sources healthy? / Has my source finished snapshotting?"

"What's the health of my environment?"

"What can I optimize to save costs?"

Notes

Connecting an MCP client to `materialize-developer`

`mz_introspection` relations need the `query` tool

mz-health-check

Materialize Developer Analysis