~ubuntu-branches/ubuntu/oneiric/postgresql-9.1/oneiric-security

constructs an index on the specified column(s) of the specified table\&. Indexes are primarily used to enhance database performance (though inappropriate use can result in slower performance)\&.

.PP

The key field(s) for the index are specified as column names, or alternatively as expressions written in parentheses\&. Multiple fields can be specified if the index method supports multicolumn indexes\&.

.PP

An index field can be an expression computed from the values of one or more columns of the table row\&. This feature can be used to obtain fast access to data based on some transformation of the basic data\&. For example, an index computed on

upper(col)

would allow the clause

WHERE upper(col) = \(aqJIM\(aq

to use an index\&.

.PP

PostgreSQL

provides the index methods B\-tree, hash, GiST, and GIN\&. Users can also define their own index methods, but that is fairly complicated\&.

.PP

When the

WHERE

clause is present, a

partial index

is created\&. A partial index is an index that contains entries for only a portion of a table, usually a portion that is more useful for indexing than the rest of the table\&. For example, if you have a table that contains both billed and unbilled orders where the unbilled orders take up a small fraction of the total table and yet that is an often used section, you can improve performance by creating an index on just that portion\&. Another possible application is to use

WHERE

with

UNIQUE

to enforce uniqueness over a subset of a table\&. See

Section 11.8, \(lqPartial Indexes\(rq, in the documentation

for more discussion\&.

.PP

The expression used in the

WHERE

clause can refer only to columns of the underlying table, but it can use all columns, not just the ones being indexed\&. Presently, subqueries and aggregate expressions are also forbidden in

WHERE\&. The same restrictions apply to index fields that are expressions\&.

.PP

All functions and operators used in an index definition must be

\(lqimmutable\(rq, that is, their results must depend only on their arguments and never on any outside influence (such as the contents of another table or the current time)\&. This restriction ensures that the behavior of the index is well\-defined\&. To use a user\-defined function in an index expression or

WHERE

clause, remember to mark the function immutable when you create it\&.

.SH "PARAMETERS"

.PP

UNIQUE

.RS 4

Causes the system to check for duplicate values in the table when the index is created (if data already exist) and each time data is added\&. Attempts to insert or update data which would result in duplicate entries will generate an error\&.

.RE

.PP

CONCURRENTLY

.RS 4

When this option is used,

PostgreSQL

will build the index without taking any locks that prevent concurrent inserts, updates, or deletes on the table; whereas a standard index build locks out writes (but not reads) on the table until it\(aqs done\&. There are several caveats to be aware of when using this option \(em see

Building Indexes Concurrently\&.

.RE

.PP

\fIname\fR

.RS 4

The name of the index to be created\&. No schema name can be included here; the index is always created in the same schema as its parent table\&. If the name is omitted,

PostgreSQL

chooses a suitable name based on the parent table\(aqs name and the indexed column name(s)\&.

.RE

.PP

\fItable\fR

.RS 4

The name (possibly schema\-qualified) of the table to be indexed\&.

.RE

.PP

\fImethod\fR

.RS 4

The name of the index method to be used\&. Choices are

100

btree,

101

hash,

102

gist, and

103

gin\&. The default method is

104

btree\&.

105

.RE

106

.PP

107

\fIcolumn\fR

108

.RS 4

109

The name of a column of the table\&.

110

.RE

111

.PP

112

\fIexpression\fR

113

.RS 4

114

An expression based on one or more columns of the table\&. The expression usually must be written with surrounding parentheses, as shown in the syntax\&. However, the parentheses can be omitted if the expression has the form of a function call\&.

115

.RE

116

.PP

117

\fIcollation\fR

118

.RS 4

119

The name of the collation to use for the index\&. By default, the index uses the collation declared for the column to be indexed or the result collation of the expression to be indexed\&. Indexes with non\-default collations can be useful for queries that involve expressions using non\-default collations\&.

120

.RE

121

.PP

122

\fIopclass\fR

123

.RS 4

124

The name of an operator class\&. See below for details\&.

125

.RE

126

.PP

127

ASC

128

.RS 4

129

Specifies ascending sort order (which is the default)\&.

130

.RE

131

.PP

132

DESC

133

.RS 4

134

Specifies descending sort order\&.

135

.RE

136

.PP

137

NULLS FIRST

138

.RS 4

139

Specifies that nulls sort before non\-nulls\&. This is the default when

140

DESC

141

is specified\&.

142

.RE

143

.PP

144

NULLS LAST

145

.RS 4

146

Specifies that nulls sort after non\-nulls\&. This is the default when

147

DESC

148

is not specified\&.

149

.RE

150

.PP

151

\fIstorage_parameter\fR

152

.RS 4

153

The name of an index\-method\-specific storage parameter\&. See

154

Index Storage Parameters

155

for details\&.

156

.RE

157

.PP

158

\fItablespace\fR

159

.RS 4

160

The tablespace in which to create the index\&. If not specified,

161

default_tablespace

162

is consulted, or

163

temp_tablespaces

164

for indexes on temporary tables\&.

165

.RE

166

.PP

167

\fIpredicate\fR

168

.RS 4

169

The constraint expression for a partial index\&.

170

.RE

171

.SS "Index Storage Parameters"

172

.PP

173

The optional

174

WITH

175

clause specifies

176

storage parameters

177

for the index\&. Each index method has its own set of allowed storage parameters\&. The B\-tree, hash and GiST index methods all accept a single parameter:

178

.PP

179

FILLFACTOR

180

.RS 4

181

The fillfactor for an index is a percentage that determines how full the index method will try to pack index pages\&. For B\-trees, leaf pages are filled to this percentage during initial index build, and also when extending the index at the right (adding new largest key values)\&. If pages subsequently become completely full, they will be split, leading to gradual degradation in the index\(aqs efficiency\&. B\-trees use a default fillfactor of 90, but any integer value from 10 to 100 can be selected\&. If the table is static then fillfactor 100 is best to minimize the index\(aqs physical size, but for heavily updated tables a smaller fillfactor is better to minimize the need for page splits\&. The other index methods use fillfactor in different but roughly analogous ways; the default fillfactor varies between methods\&.

182

.RE

183

.PP

184

GIN indexes accept a different parameter:

185

.PP

186

FASTUPDATE

187

.RS 4

188

This setting controls usage of the fast update technique described in

189

Section 54.3.1, \(lqGIN Fast Update Technique\(rq, in the documentation\&. It is a Boolean parameter:

190

191

enables fast update,

192

OFF

193

disables it\&. (Alternative spellings of

194

195

and

196

OFF

197

are allowed as described in

198

Section 18.1, \(lqSetting Parameters\(rq, in the documentation\&.) The default is

199

ON\&.

200

.if n \{\

201

.sp

202

.\}

203

.RS 4

204

.it 1 an-trap

205

.nr an-no-space-flag 1

206

.nr an-break-flag 1

207

.br

208

.ps +1

209

\fBNote\fR

210

.ps -1

211

.br

212

Turning

213

FASTUPDATE

214

off via

215

ALTER INDEX

216

prevents future insertions from going into the list of pending index entries, but does not in itself flush previous entries\&. You might want to

217

VACUUM

218

the table afterward to ensure the pending list is emptied\&.

219

.sp .5v

220

.RE

221

.RE

222

.SS "Building Indexes Concurrently"

223

.\" index: building concurrently

224

.PP

225

Creating an index can interfere with regular operation of a database\&. Normally

226

PostgreSQL

227

locks the table to be indexed against writes and performs the entire index build with a single scan of the table\&. Other transactions can still read the table, but if they try to insert, update, or delete rows in the table they will block until the index build is finished\&. This could have a severe effect if the system is a live production database\&. Very large tables can take many hours to be indexed, and even for smaller tables, an index build can lock out writers for periods that are unacceptably long for a production system\&.

228

.PP

229

PostgreSQL

230

supports building indexes without locking out writes\&. This method is invoked by specifying the

231

CONCURRENTLY

232

option of

233

CREATE INDEX\&. When this option is used,

234

PostgreSQL

235

must perform two scans of the table, and in addition it must wait for all existing transactions that could potentially use the index to terminate\&. Thus this method requires more total work than a standard index build and takes significantly longer to complete\&. However, since it allows normal operations to continue while the index is built, this method is useful for adding new indexes in a production environment\&. Of course, the extra CPU and I/O load imposed by the index creation might slow other operations\&.

236

.PP

237

In a concurrent index build, the index is actually entered into the system catalogs in one transaction, then the two table scans occur in a second and third transaction\&. If a problem arises while scanning the table, such as a uniqueness violation in a unique index, the

238

CREATE INDEX

239

command will fail but leave behind an

240

\(lqinvalid\(rq

241

index\&. This index will be ignored for querying purposes because it might be incomplete; however it will still consume update overhead\&. The

242

psql\ed

243

command will report such an index as

244

INVALID:

245

.sp

246

.if n \{\

247

.RS 4

248

.\}

249

.nf

250

postgres=# \ed tab

251

Table "public\&.tab"

252

Column | Type | Modifiers

253

\-\-\-\-\-\-\-\-+\-\-\-\-\-\-\-\-\-+\-\-\-\-\-\-\-\-\-\-\-

254

col | integer |

255

Indexes:

256

"idx" btree (col) INVALID

257

.fi

258

.if n \{\

259

.RE

260

.\}

261

.sp

262

The recommended recovery method in such cases is to drop the index and try again to perform

263

CREATE INDEX CONCURRENTLY\&. (Another possibility is to rebuild the index with

264

REINDEX\&. However, since

265

REINDEX

266

does not support concurrent builds, this option is unlikely to seem attractive\&.)

267

.PP

268

Another caveat when building a unique index concurrently is that the uniqueness constraint is already being enforced against other transactions when the second table scan begins\&. This means that constraint violations could be reported in other queries prior to the index becoming available for use, or even in cases where the index build eventually fails\&. Also, if a failure does occur in the second scan, the

269

\(lqinvalid\(rq

270

index continues to enforce its uniqueness constraint afterwards\&.

271

.PP

272

Concurrent builds of expression indexes and partial indexes are supported\&. Errors occurring in the evaluation of these expressions could cause behavior similar to that described above for unique constraint violations\&.

273

.PP

274

Regular index builds permit other regular index builds on the same table to occur in parallel, but only one concurrent index build can occur on a table at a time\&. In both cases, no other types of schema modification on the table are allowed meanwhile\&. Another difference is that a regular

275

CREATE INDEX

276

command can be performed within a transaction block, but

277

CREATE INDEX CONCURRENTLY

278

cannot\&.

279

.SH "NOTES"

280

.PP

281

See

282

Chapter 11, Indexes, in the documentation

283

for information about when indexes can be used, when they are not used, and in which particular situations they can be useful\&.

284

.PP

285

Currently, only the B\-tree, GiST and GIN index methods support multicolumn indexes\&. Up to 32 fields can be specified by default\&. (This limit can be altered when building

286

PostgreSQL\&.) Only B\-tree currently supports unique indexes\&.

287

.PP

288

289

operator class

290

can be specified for each column of an index\&. The operator class identifies the operators to be used by the index for that column\&. For example, a B\-tree index on four\-byte integers would use the

291

int4_ops

292

class; this operator class includes comparison functions for four\-byte integers\&. In practice the default operator class for the column\(aqs data type is usually sufficient\&. The main point of having operator classes is that for some data types, there could be more than one meaningful ordering\&. For example, we might want to sort a complex\-number data type either by absolute value or by real part\&. We could do this by defining two operator classes for the data type and then selecting the proper class when making an index\&. More information about operator classes is in

293

Section 11.9, \(lqOperator Classes and Operator Families\(rq, in the documentation

294

and in

295

Section 35.14, \(lqInterfacing Extensions To Indexes\(rq, in the documentation\&.

296

.PP

297

For index methods that support ordered scans (currently, only B\-tree), the optional clauses

298

ASC,

299

DESC,

300

NULLS FIRST, and/or

301

NULLS LAST

302

can be specified to modify the sort ordering of the index\&. Since an ordered index can be scanned either forward or backward, it is not normally useful to create a single\-column

303

DESC

304

index \(em that sort ordering is already available with a regular index\&. The value of these options is that multicolumn indexes can be created that match the sort ordering requested by a mixed\-ordering query, such as

305

SELECT \&.\&.\&. ORDER BY x ASC, y DESC\&. The

306

NULLS

307

options are useful if you need to support

308

\(lqnulls sort low\(rq

309

behavior, rather than the default

310

\(lqnulls sort high\(rq, in queries that depend on indexes to avoid sorting steps\&.

311

.PP

312

For most index methods, the speed of creating an index is dependent on the setting of

313

maintenance_work_mem\&. Larger values will reduce the time needed for index creation, so long as you don\(aqt make it larger than the amount of memory really available, which would drive the machine into swapping\&. For hash indexes, the value of

314

effective_cache_size

315

is also relevant to index creation time:

316

PostgreSQL

317

will use one of two different hash index creation methods depending on whether the estimated index size is more or less than

318

\fIeffective_cache_size\fR\&. For best results, make sure that this parameter is also set to something reflective of available memory, and be careful that the sum of

319

\fImaintenance_work_mem\fR

320

and

321

\fIeffective_cache_size\fR

322

is less than the machine\(aqs RAM less whatever space is needed by other programs\&.

323

.PP

324

Use

325

DROP INDEX (\fBDROP_INDEX\fR(7))

326

to remove an index\&.

327

.PP

328

Prior releases of

329

PostgreSQL

330

also had an R\-tree index method\&. This method has been removed because it had no significant advantages over the GiST method\&. If

331

USING rtree

332

is specified,

333

CREATE INDEX

334

will interpret it as

335

USING gist, to simplify conversion of old databases to GiST\&.

336

.SH "EXAMPLES"

337

.PP

338

To create a B\-tree index on the column

339

title

340

in the table

341

films:

342

.sp

343

.if n \{\

344

.RS 4

345

.\}

346

.nf

347

CREATE UNIQUE INDEX title_idx ON films (title);

348

.fi

349

.if n \{\

350

.RE

351

.\}

352

.PP

353

To create an index on the expression

354

lower(title), allowing efficient case\-insensitive searches:

355

.sp

356

.if n \{\

357

.RS 4

358

.\}

359

.nf

360

CREATE INDEX ON films ((lower(title)));

361

.fi

362

.if n \{\

363

.RE

364

.\}

365

.sp

366

(In this example we have chosen to omit the index name, so the system will choose a name, typically

367

films_lower_idx\&.)

368

.PP

369

To create an index with non\-default collation:

370

.sp

371

.if n \{\

372

.RS 4

373

.\}

374

.nf

375

CREATE INDEX title_idx_german ON films (title COLLATE "de_DE");

376

.fi

377

.if n \{\

378

.RE

379

.\}

380

.PP

381

To create an index with non\-default sort ordering of nulls:

382

.sp

383

.if n \{\

384

.RS 4

385

.\}

386

.nf

387

CREATE INDEX title_idx_nulls_low ON films (title NULLS FIRST);

388

.fi

389

.if n \{\

390

.RE

391

.\}

392

.PP

393

To create an index with non\-default fill factor:

394

.sp

395

.if n \{\

396

.RS 4

397

.\}

398

.nf

399

CREATE UNIQUE INDEX title_idx ON films (title) WITH (fillfactor = 70);

400

.fi

401

.if n \{\

402

.RE

403

.\}

404

.PP

405

To create a

406

GIN

407

index with fast updates disabled:

408

.sp

409

.if n \{\

410

.RS 4

411

.\}

412

.nf

413

CREATE INDEX gin_idx ON documents_table USING gin (locations) WITH (fastupdate = off);

414

.fi

415

.if n \{\

416

.RE

417

.\}

418

.PP

419

To create an index on the column

420

code

421

in the table

422

films

423

and have the index reside in the tablespace

424

indexspace:

425

.sp

426

.if n \{\

427

.RS 4

428

.\}

429

.nf

430

CREATE INDEX code_idx ON films (code) TABLESPACE indexspace;

431

.fi

432

.if n \{\

433

.RE

434

.\}

435

.PP

436

To create a GiST index on a point attribute so that we can efficiently use box operators on the result of the conversion function:

437

.sp

438

.if n \{\

439

.RS 4

440

.\}

441

.nf

442

CREATE INDEX pointloc

443

ON points USING gist (box(location,location));

444

SELECT * FROM points

445

WHERE box(location,location) && \(aq(0,0),(1,1)\(aq::box;

446

.fi

447

.if n \{\

448

.RE

449

.\}

450

.PP

451

To create an index without locking out writes to the table:

452

.sp

453

.if n \{\

454

.RS 4

455

.\}

456

.nf

457

CREATE INDEX CONCURRENTLY sales_quantity_index ON sales_table (quantity);

458

.fi

459

.if n \{\

460

.RE

461

.\}

462

.SH "COMPATIBILITY"

463

.PP

464

CREATE INDEX

465

is a

466

PostgreSQL

467

language extension\&. There are no provisions for indexes in the SQL standard\&.

468

.SH "SEE ALSO"

469

ALTER INDEX (\fBALTER_INDEX\fR(7)), DROP INDEX (\fBDROP_INDEX\fR(7))

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