Choose Big Data Solution

Help users and agents select the right MATLAB tool for large tabular data in file-based formats (CSV, Parquet, delimited text, spreadsheets, MDF). The skill encodes a decision flowchart — recommend one clear path, not a menu of options.

When to Use

• User or agent has large tabular data in file-based formats (CSV, Parquet, delimited text, spreadsheets, MDF)
• User or agent says data is "large", "big", or "huge" (even without a specific size)
• User or agent hits an out-of-memory error with readtable, parquetread, or similar
• User or agent has multiple tabular files to process as a single dataset
• User or agent has multiple tabular files to process independently (per-file)
• User or agent asks how to scale up an existing workflow on tabular file data
• User or agent asks about datastores, tall arrays, or mapreduce
• User or agent needs to build a custom datastore class for a proprietary or non-standard format
• User or agent has large XML, JSON, HTML, or Word document files — readtable supports these formats but the built-in datastores do not. Scaling these requires a custom datastore (see "Formats Without a Built-in Datastore" and "Custom Datastores" sections below)

When NOT to Use

• Single file that fits in memory (file-size-to-RAM ratio < 0.5) — see Pre-Flight Check and matlab-import-export-data skill
• User or agent is working with MAT files — matfile provides partial I/O for large .mat files, different workflow
• User or agent is working with databases (SQL, ODBC) — use Database Toolbox. See the matlab-use-database and matlab-use-duckdb skills in the reporting-and-database-access category
• User or agent needs GPU acceleration — different domain
• Deep learning training workflows — datastore is correct for data loading, but use minibatchqueue for batching (not tall or transform)

Pre-Flight Check

ALWAYS run this check BEFORE recommending datastore or tall array patterns. Estimate the file-size-to-available-RAM ratio (accounting for in-memory expansion of the file format).

Condition	Route	Rationale
Single file, ratio < 0.5	Native MATLAB I/O (readtable, parquetread)	Fits in memory; datastore/tall adds unnecessary complexity
Single file, ratio ≥ 0.5, OR user/agent reports OOM	Continue to Decision Flowchart below	Data may not fit in memory
Multiple files	Continue to Decision Flowchart below	Datastore patterns provide unified multi-file access
Size unknown and user/agent describes data as "large", "big", or "huge"	Continue to Decision Flowchart below	Assume large until proven otherwise

If the data fits in memory (single file, ratio < 0.5): recommend native I/O and stop. Mention that datastore and tall array patterns exist if the data grows beyond memory in the future, but do not implement them now.

Decision Flowchart

Follow this flowchart strictly. Present ONE recommended path, not multiple alternatives. Only mention alternatives if the situation is ambiguous.

Is the data described as "large" or causing OOM?
│
├── YES
│   │
│   ├── Is the goal to process all data as ONE continuous dataset?
│   │   │
│   │   └── YES → Datastore + Tall Arrays
│   │             Choose datastore by format:
│   │               CSV/delimited text → tabularTextDatastore
│   │               Parquet            → parquetDatastore
│   │               Excel (.xlsx/.xls) → spreadsheetDatastore
│   │               MDF (.mf4/.mdf)    → mdfDatastore (requires Vehicle Network Toolbox)
│   │               Other formats      → Custom Datastore
│   │
│   └── Is the goal to process each unit INDEPENDENTLY?
│       │
│       ├── One read = one FILE
│       │     CSV/delimited text → tabularTextDatastore
│       │     Excel (.xlsx/.xls) → spreadsheetDatastore
│       │     Other formats      → Custom Datastore or fileDatastore
│       │
│       └── One read = one ROW GROUP
│             Parquet → parquetDatastore
│
└── NO / UNCLEAR
    └── STOP. Ask: (1) what is the file format? (2) do you need to process
        all data as one dataset, or each file/unit independently?
        Do NOT show code until these are answered.

Optional (requires Parallel Computing Toolbox):
├── Speed up tall arrays locally → Open a parallel pool
├── Speed up readall on transforms → UseParallel
└── Speed up tall arrays on Hadoop/Spark → mapreducer (also requires MATLAB Parallel Server)

Critical rules:

• When the processing goal is unclear (continuous vs per-file), STOP and ask before writing any code. Do not show code examples, do not generate scripts, do not demonstrate both approaches. Ask which applies and wait for the answer. The correct response to ambiguity is a short clarifying question, not a menu of options with code for each.
• When data is described as "large", NEVER lead with readtable or parquetread
• For per-file or per-row-group processing, NEVER suggest tall arrays (tall merges all data into one dataset and has no concept of file or row group boundaries)
• For Parquet, the natural independent unit is the row group, not the file — parquetDatastore defaults to ReadSize = "rowgroup". Do NOT force ReadSize = "file" on a Parquet datastore unless the workflow truly needs whole-file granularity
• For parallelism, recommend parallel pool FIRST; mapreducer is only for Hadoop/Spark
• Do NOT present manual chunking (while/read loops) as the first option — tall arrays handle chunking automatically
• Do NOT present multiple options — follow the flowchart and recommend ONE clear path

Datastore + Tall Arrays (continuous dataset)

Use when: processing one large file OR multiple files as a single dataset. Use the datastore matching the format (see Decision Flowchart above).

% Single file
ds = tabularTextDatastore("largedata.csv");
tt = tall(ds);

% Multiple files
ds = tabularTextDatastore("data/*.csv");
tt = tall(ds);

% For example, compute statistics with the tall array and gather results.
% Tall handles chunking automatically. Specify DataVars to avoid errors
% on non-numeric columns.
result = groupsummary(tt, "GroupVar", {"mean", "std"}, "NumericVar");
result = gather(result);

Key points:

• Most common functions work on tall: mean, std, min, max, groupsummary, sortrows, topkrows
• Combine multiple gathers: [a, b] = gather(tallA, tallB)

DuckDB alternative (R2026a+): When the goal is to filter, aggregate, deduplicate, or sample a large file down to a small in-memory result, a single DuckDB query may be faster than datastore + tall. DuckDB queries CSV/Parquet/JSON files directly without loading them into memory. Requires Database Toolbox.

conn = duckdb();
result = fetch(conn, "SELECT * FROM read_csv_auto('large.csv') WHERE Region = 'West'");
close(conn);

For complex DuckDB workflows, see the matlab-use-duckdb skill in the reporting-and-database-access category.

Migrating from readtable (OOM on large files):

% Before:
T = readtable("large.csv");
stats = groupsummary(T, "Category", "mean");

% After:
ds = tabularTextDatastore("large.csv");
tt = tall(ds);
stats = gather(groupsummary(tt, "Category", "mean"));

tabularTextDatastore uses a stricter textscan-based parser. Key gotchas:

• TextType must be set at creation time — read-only after construction
• TrimNonNumeric unsupported — read as %q, strip on the tall array
• %f errors on non-numeric content — read as %q, convert with str2double
• ExtraColumnsRule does not exist — append "%*[^\r\n]" to TextscanFormats
• detectImportOptions does not apply — configure via datastore properties directly

See references/readtable-to-datastore-migration.md for the full property mapping.

Migrating from parquetread: Replace with parquetDatastore + tall. Types are preserved exactly — no format specifier issues. All parquetread options map 1:1 to datastore properties. See references/parquetread-to-datastore-migration.md.

Datastore + Transform (per-file)

Use when: each file in a folder should be processed independently (per-file statistics, per-file transformations, file-level aggregation).

% Set up datastore to read one file at a time
ds = tabularTextDatastore("data/*.csv");
ds.ReadSize = "file";

% For example, compute statistics by transforming the datastore with a custom function.
tds = transform(ds, @computeFileStats);
results = readall(tds);

function out = computeFileStats(data)
    numVars = vartype("numeric");
    out = table( ...
        min(data{:, numVars}, [], 1, "omitmissing"), ...
        max(data{:, numVars}, [], 1, "omitmissing"), ...
        mean(data{:, numVars}, 1, "omitmissing"), ...
        VariableNames=["Min", "Max", "Mean"]);
end

For Excel files, use spreadsheetDatastore instead of tabularTextDatastore. spreadsheetDatastore defaults to ReadSize = "file" so no override is needed. For per-sheet processing (e.g., statistics per worksheet), set ReadSize = "sheet" so each read returns exactly one sheet's data.

Key points:

• For tabularTextDatastore, set ReadSize = "file" so each read returns exactly one file's data. The default ReadSize is a row count, which can split a single file across multiple reads but it will not go across file boundaries
• transform applies a function to each read — the datastore output contains only the transformed results, not the original data. The transform function does not need to return the same number of rows as its input (e.g., computing the mean of each variable produces a single row per read)
• readall on the transformed datastore collects all per-file results
• Use "omitmissing" for missing-data flags (R2023a+). On R2022b and earlier, use "omitnan" instead.

Do NOT use tall arrays for per-file processing. Tall arrays treat all files as one continuous dataset — they have no concept of file boundaries.

Datastore + Transform (per-row-group, Parquet)

Use when: a Parquet dataset is partitioned by row group (e.g., one row group per item, sensor, region, or time bucket) and each row group must be processed independently. Row groups are the natural unit of independence in Parquet — data from different row groups should not be mixed when the partitioning encodes a meaningful grouping.

% parquetDatastore defaults ReadSize to "rowgroup" — one read = one row group.
% Do NOT set ReadSize = "file": that would mix row groups within the same file.
pds = parquetDatastore("data/");

% For example, compute statistics by transforming the datastore with a custom function.
tds = transform(pds, @computeRowGroupStats);
results = readall(tds);

function out = computeRowGroupStats(data)
    out = table( ...
        unique(data.item), ...
        mean(data.val, "omitmissing"), ...
        std(data.val, "omitmissing"), ...
        VariableNames=["Item", "Mean", "Std"]);
end

Key points:

• parquetDatastore defaults to ReadSize = "rowgroup". Each read returns exactly one row group — leave the default
• A row group is the smallest independently readable unit inside a Parquet file. Writers commonly assign one row group per partition key (item, sensor, day), so per-row-group processing preserves those boundaries

Do NOT set ReadSize = "file" on a Parquet datastore for this workflow — it merges all row groups in a file into a single read and silently mixes data that was meant to stay separate. Use file-level granularity only when each file already contains exactly one logical group.

Parallelizing readall (transform workflows)

The readall call in both per-file and per-row-group transform workflows supports parallel execution when Parallel Computing Toolbox is installed and licensed. Suggest this only when the toolbox is available:

results = readall(tds, UseParallel=true);

• UseParallel=true parallelizes reads across workers in an open parallel pool. If no pool is open, MATLAB opens one automatically
• Only suggest UseParallel if Parallel Computing Toolbox is available

Parallel Pool (local parallelism for tall arrays)

Use when: the goal is to speed up a tall array workflow with local cores. Only recommend if Parallel Computing Toolbox is installed and licensed.

% Start a thread or process pool
parpool("Threads");
% parpool("Processes");

ds = tabularTextDatastore("data/*.csv");
tt = tall(ds);
result = gather(mean(tt, "omitmissing"));

Tall array computations automatically distribute across available workers when a parallel pool is open. No code changes needed beyond opening the pool.

Do NOT suggest parpool or parallel pool workflows unless Parallel Computing Toolbox is available — parpool errors without it.

Mapreducer (Hadoop/Spark only)

Use ONLY when the user or agent explicitly has a Hadoop or Spark environment. Requires both Parallel Computing Toolbox and MATLAB Parallel Server.

mr = mapreducer(cluster);
ds = tabularTextDatastore("hdfs:///data/*.csv");
tt = tall(ds);
result = gather(mean(tt, "omitmissing"));

Do NOT recommend mapreducer for local parallel processing — a parallel pool is simpler and sufficient for local multi-core execution.

Do NOT suggest mapreducer for Hadoop/Spark execution unless the user or agent has Parallel Computing Toolbox and a Hadoop or Spark environment with MATLAB Parallel Server installed and licensed.

Key Functions

Function	Purpose	When to Use
tabularTextDatastore	Multiple reads per file for CSV/text	Large CSV/text files
parquetDatastore	Multiple reads per file for Parquet	Large Parquet files
spreadsheetDatastore	Multiple reads per file for Excel	Large .xlsx/.xls files
mdfDatastore	Multiple reads per file for MDF	Large .mf4/.mdf files (requires Vehicle Network Toolbox)
tall	Lazy evaluation over a datastore	Continuous dataset processing
gather	Execute deferred tall computations	Collect results into memory
transform	Apply function to each datastore read	Per-file (text) or per-row-group (Parquet) processing
groupsummary	Grouped statistics (tall-compatible)	Aggregation by category
parpool	Open parallel worker pool	Speed up tall computations
mapreducer	Connect to Hadoop/Spark cluster	Hadoop/Spark environments only

Formats Without a Built-in Datastore

Built-in datastores (such as tabularTextDatastore, parquetDatastore, spreadsheetDatastore, mdfDatastore) do NOT support JSON, XML, HTML, or Word documents. If the user or agent has an OOM error with readtable on one of these formats, do NOT recommend converting to Parquet/CSV first — readtable itself would OOM during the conversion, creating a circular dependency.

Why not fileDatastore with readtable? A common instinct is to use fileDatastore(@readtable) — but if readtable OOMs on the file, wrapping it in fileDatastore does not help because the read function still loads the entire file into memory.

Instead, recommend building a custom datastore that performs multiple reads per file (chunked reading), processing it in manageable pieces without loading it entirely into memory (e.g., reading N lines of XML at a time, or parsing JSON arrays incrementally). A fileDatastore with such a read function is one option; implementing the full matlab.io.Datastore interface is another. See the "Custom Datastores" section below for implementation guidance.

A custom datastore can then be used with tall or transform just like the built-in datastores.

Custom Datastores

If a file format is not supported by one of the built-in datastores, build a custom datastore by subclassing matlab.io.Datastore. This applies to:

• Proprietary binary formats
• Non-standard text formats (custom log files, sensor dumps)
• Formats requiring multiple reads per file (large XML, JSON, HTML)
• Any format needing custom iteration, partitioning, or stateful reads

Before building a custom datastore, check whether fileDatastore with a custom ReadFcn is sufficient (one file = one read, each file fits in memory, no state needed). Set UniformRead=true at construction so readall returns a concatenated table (read-only after construction). For partial reads within a file, fileDatastore with ReadMode="partialfile" supports chunked iteration (see references/custom-datastore/implementation.md for the required 3-output ReadFcn signature). Only build a full custom datastore class when those simpler patterns are insufficient.

For full implementation guidance, read references/custom-datastore/implementation.md. It covers:

• Decision flowchart (fileDatastore vs custom class)
• Requirement gathering (mixins, FileSet vs BlockedFileSet)
• Complete code patterns (FileSet and BlockedFileSet templates)
• Common mistakes and conventions
• Testing guidelines (references/custom-datastore/testing-guidelines.md)
• Mixin API reference (references/custom-datastore/mixin-decision-tree.md)

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