~ubuntu-branches/ubuntu/wily/sqlite3/wily

onfocus="entersearch()" onblur="leavesearch()" style="width:24ex;padding:1px 1ex; border:solid white 1px; font-size:0.9em ; font-style:italic;color:#044a64;" value="Search SQLite Docs...">

112

113

</form>

114

</div>

115

</table>

116

</div></div></div></div>

117

</td></tr></table>

118

119

120

121

122

123

124

125

126

127

EXPLAIN QUERY PLAN

128

</div>

129

130

Table Of Contents</div>

131

<div id=toc> <div style="margin-left:6ex"><a href="#section_1">1. The EXPLAIN QUERY PLAN Command</a></div><div style="margin-left:12ex"><a href="#section_1_1">1.1. Table and Index Scans</a></div><div style="margin-left:12ex"><a href="#section_1_2">1.2. Temporary Sorting B-Trees</a></div><div style="margin-left:12ex"><a href="#section_1_3">1.3. Subqueries</a></div><div style="margin-left:12ex"><a href="#section_1_4">1.4. Compound Queries</a></div><div style="margin-left:6ex"><a href="#section_2">2. Sample Code</a></div> </div>

132

133

134

135

<h1 id="section_1">1. The EXPLAIN QUERY PLAN Command</h1>

136

137

138

<b>Warning: The data returned by the EXPLAIN QUERY PLAN command is

139

intended for interactive debugging only. It may change dramatically between

140

SQLite releases. Applications should not depend on the results of an EXPLAIN

141

QUERY PLAN command.</b>

142

143

<p>The <a href="lang_explain.html">EXPLAIN QUERY PLAN</a> SQL command is used to obtain a high-level

144

description of the strategy or plan that SQLite uses to implement a specific

145

SQL query. Most significantly, it reports on the way in which the query uses

146

database indices. In interpreting and using this information to optimize

147

database schemas and queries, users might find the documents describing how

148

SQLite <a href="queryplanner.html">plans</a> and <a href="optoverview.html">optimizes</a> queries useful.

149

150

<p>An EXPLAIN QUERY PLAN command returns zero or more rows of four columns

151

each. The column names are "selectid", "order", "from", "detail". The

152

first three columns contain an integer value. The final

153

column, "detail", contains a text value which carries most of

154

the useful information.

155

156

<p>EXPLAIN QUERY PLAN is most useful on a SELECT statement,

157

but may also be appear with other statements that read data from database

158

tables (e.g. UPDATE, DELETE, INSERT INTO ... SELECT).

159

160

<h2 id="section_1_1">1.1. Table and Index Scans</h2>

161

162

<p>

163

When processing a SELECT (or other) statement, SQLite may retrieve data from

164

database tables in a variety of ways. It may scan through all the records in

165

a table (a full-table scan), scan a contiguous subset of the records in a

166

table based on the rowid index, scan a contiguous subset of the entries in a

167

database <a href="lang_createtable.html">index</a>, or use a combination of the above strategies

168

in a single scan. The various ways in which SQLite may retrieve data from a

169

table or index are described in detail <a href="queryplanner.html#searching">here</a>.

170

171

<p>

172

For each table read by the query, the output of EXPLAIN QUERY

173

PLAN includes a record for which the value in the "detail" column begins

174

with either "SCAN" or "SEARCH". "SCAN" is used for a full-table scan,

175

including cases where SQLite iterates through all records in a table

176

in an order defined by an index. "SEARCH" indicates that only a subset of

177

the table rows are visited. Each SCAN or SEARCH record includes the

178

following information:

179

180

<ul>

181

<li> The name of the table data is read from.

182

<li> Whether or not an index or <a href="optoverview.html#autoindex">automatic index</a> is used.

183

<li> Whether or not the <a href="queryplanner.html#covidx">covering index</a> optimization applies.

184

<li> Which terms of the WHERE clause are used for indexing.

185

<li> The estimated number of rows that SQLite will visit.

186

</ul>

187

188

<p>

189

For example, the following EXPLAIN QUERY PLAN command operates on a SELECT

190

statement that is implemented by performing a full-table scan on table t1:

191

<div class=codeblock style="margin:0 15ex"><table width=100% style="border:1px solid #80a796;padding:0 1ex;background-color:#EEEEEE"><tr><td><pre style="font-size:1.1em">sqlite> EXPLAIN QUERY PLAN SELECT a, b FROM t1 WHERE a=1;

192

0|0|0|SCAN TABLE t1 (~100000 rows)

193

</table></div>

194

195

<p>

196

The example above shows

197

SQLite estimating that the full-table scan will visit approximately

198

100,000 records. If the query were able to use an index, then the

199

SCAN/SEARCH record would include the name of the index and, for a

200

SEARCH record, an indication of how the subset of rows visited is

201

identified. For example:

202

203

sqlite> EXPLAIN QUERY PLAN SELECT a, b FROM t1 WHERE a=1;

204

0|0|0|SEARCH TABLE t1 USING INDEX i1 (a=?) (~10 rows)

205

</table></div>

206

207

<p>

208

The previous example, SQLite uses index "i1" to optimize

209

a WHERE clause term of the form (a=?) - in this case "a=1". SQLite

210

estimates that about 10 records will match the "a=1" term.

211

The previous example could not use a <a href="queryplanner.html#covidx">covering index</a>, but the following

212

example can, and that fact is reflected in the output:

213

214

sqlite> EXPLAIN QUERY PLAN SELECT a, b FROM t1 WHERE a=1;

215

0|0|0|SEARCH TABLE t1 USING COVERING INDEX i2 (a=?) (~10 rows)

216

</table></div>

217

218

<p>

219

All joins in SQLite are <a href="optoverview.html#table_order">implemented using nested scans</a>. When a

220

SELECT query that features a join is analyzed using EXPLAIN QUERY PLAN, one

221

SCAN or SEARCH record is output for each nested loop. For example:

222

223

0|0|0|SEARCH TABLE t1 USING COVERING INDEX i2 (a=? AND b>?) (~3 rows)

224

0|1|1|SCAN TABLE t2 (~1000000 rows)

225

</table></div>

226

227

<p>

228

The second column of output (column "order") indicates the nesting order. In

229

this case, the scan of table t1 using index i2 is the outer loop (order=0)

230

and the full-table scan of table t2 (order=1) is the inner loop. The third

231

column (column "from"), indicates the position in the FROM clause of the

232

SELECT statement that the table associated with each scan occurs in. In the

233

case above, table t1 occupies the first position in the FROM clause, so the

234

value of column "from" is 0 in the first record. Table t2 is in the

235

second position, so the "from" column for the corresponding SCAN record is

236

set to 1. In the following example, the positions of t1 and t2 in the FROM

237

clause of the SELECT are reversed. The query strategy remains the same, but

238

the values in the "from" column of the output are adjusted accordingly.

239

240

0|0|1|SEARCH TABLE t1 USING COVERING INDEX i2 (a=? AND b>?) (~3 rows)

241

0|1|0|SCAN TABLE t2 (~1000000 rows)

242

</table></div>

243

244

<p>

245

In the example above, SQLite estimates that the outer loop scan will visit

246

approximately 3 rows, and that the inner loop will visit

247

approximately 1,000,000. If

248

you observe that SQLite's estimates are wildly inaccurate (and appear to be

249

causing it to generate sub-optimal query plans), your queries may benefit

250

from running the <a href="lang_analyze.html">ANALYZE</a> command on the database.

251

252

<p>

253

If the WHERE clause of a query contains an OR expression, then SQLite might

254

use the <a href="queryplanner.html#or_in_where">"OR by union"</a> strategy (also described

255

<a href="optoverview.html#or_opt">here</a>). In this case there will be two SEARCH records, one

256

for each index, with the same values in both the "order" and "from" columns.

257

For example:

258

259

sqlite> EXPLAIN QUERY PLAN SELECT * FROM t1 WHERE a=1 OR b=2;

260

0|0|0|SEARCH TABLE t1 USING COVERING INDEX i2 (a=?) (~10 rows)

261

0|0|0|SEARCH TABLE t1 USING INDEX i3 (b=?) (~10 rows)

262

</table></div>

263

264

<h2 id="section_1_2">1.2. Temporary Sorting B-Trees</h2>

265

266

<p>

267

If a SELECT query contains an ORDER BY, GROUP BY or DISTINCT clause,

268

SQLite may need to use a temporary b-tree structure to sort the output

269

rows. Or, it might <a href="queryplanner.html#sorting">use an index</a>. Using an index is

270

almost always much more efficient than performing a sort.

271

If a temporary b-tree is required, a record is added to the EXPLAIN

272

QUERY PLAN output with the "detail" field set to a string value of

273

the form "USE TEMP B-TREE FOR xxx", where xxx is one of "ORDER BY",

274

"GROUP BY" or "DISTINCT". For example:

275

276

277

0|0|0|SCAN TABLE t2 (~1000000 rows)

278

0|0|0|USE TEMP B-TREE FOR ORDER BY

279

</table></div>

280

281

<p>

282

In this case using the temporary b-tree can be avoided by creating an index

283

on t2(c), as follows:

284

285

286

sqlite> EXPLAIN QUERY PLAN SELECT c, d FROM t2 ORDER BY c;

287

0|0|0|SCAN TABLE t2 USING INDEX i4 (~1000000 rows)

288

</table></div>

289

290

<h2 id="section_1_3">1.3. Subqueries</h2>

291

292

<p>

293

In all the examples above, the first column (column "selectid") is always

294

set to 0. If a query contains sub-selects, either as part of the FROM

295

clause or as part of SQL expressions, then the output of EXPLAIN QUERY

296

PLAN also includes a report for each sub-select. Each sub-select is assigned

297

a distinct, non-zero "selectid" value. The top-level SELECT statement is

298

always assigned the selectid value 0. For example:

299

300

301

0|0|0|SCAN TABLE t2 (~1000000 rows)

302

0|0|0|EXECUTE SCALAR SUBQUERY 1

303

1|0|0|SEARCH TABLE t1 USING COVERING INDEX i2 (a=?) (~10 rows)

304

0|0|0|EXECUTE CORRELATED SCALAR SUBQUERY 2

305

2|0|0|SEARCH TABLE t1 USING INDEX i3 (b=?) (~10 rows)

306

</table></div>

307

308

<p>

309

The example above contains a pair of scalar subqueries assigned selectid

310

values 1 and 2. As well as a SCAN record, there are also 2 "EXECUTE"

311

records associated with the top level subquery (selectid 0), indicating

312

that subqueries 1 and 2 are executed by the top level query in a scalar

313

context. The CORRELATED qualifier present in the EXECUTE record associated

314

with scalar subquery 2 indicates that the query must be run separately

315

for each row visited by the top level query. Its absence in the record

316

associated with subquery 1 means that the subquery is only run once and

317

the result cached. In other words, subquery 2 may be more performance

318

critical, as it may be run many times whereas subquery 1 is only ever run

319

once.

320

321

<p>

322

Unless the <a href="optoverview.html#flattening">flattening optimization</a> is applied, if a subquery appears in

323

the FROM clause of a SELECT statement, SQLite executes the subquery and

324

stores the results in a temporary table. It then uses the contents of the

325

temporary table in place of the subquery to execute the parent query. This

326

is shown in the output of EXPLAIN QUERY PLAN by substituting a

327

"SCAN SUBQUERY" record for the "SCAN TABLE" record that normally appears

328

for each element in the FROM clause. For example:

329

330

331

1|0|0|SCAN TABLE t1 USING COVERING INDEX i2 (~1000000 rows)

332

0|0|0|SCAN SUBQUERY 1 (~1000000 rows)

333

0|0|0|USE TEMP B-TREE FOR GROUP BY

334

</table></div>

335

336

<p>

337

If the <a href="optoverview.html#flattening">flattening optimization</a> is used on a subquery in the FROM clause

338

of a SELECT statement, then the output of EXPLAIN QUERY PLAN reflects this.

339

For example, in the following there is no "SCAN SUBQUERY" record even though

340

there is a subquery in the FROM clause of the top level SELECT. Instead, since

341

the flattening optimization does apply in this case, the EXPLAIN QUERY PLAN

342

report shows that the top level query is implemented using a nested loop join

343

of tables t1 and t2.

344

345

346

0|0|0|SEARCH TABLE t2 USING INDEX i4 (c=?) (~10 rows)

347

0|1|1|SCAN TABLE t1 (~1000000 rows)

348

</table></div>

349

350

<h2 id="section_1_4">1.4. Compound Queries</h2>

351

352

<p>

353

Each component query of a <a href="lang_select.html#compound">compound query</a> (UNION, UNION ALL, EXCEPT or

354

INTERSECT) is assigned its own selectid and reported on separately. A

355

single record is output for the parent (compound query) identifying the

356

operation, and whether or not a temporary b-tree is used to implement

357

it. For example:

358

359

360

1|0|0|SCAN TABLE t1 (~1000000 rows)

361

2|0|0|SCAN TABLE t2 (~1000000 rows)

362

0|0|0|COMPOUND SUBQUERIES 1 AND 2 USING TEMP B-TREE (UNION)

363

</table></div>

364

365

<p>

366

The "USING TEMP B-TREE" clause in the above output indicates that a

367

temporary b-tree structure is used to implement the UNION of the results

368

of the two sub-selects. If the temporary b-tree were not required, as

369

in the following example, the clause is not present.

370

371

372

1|0|0|SCAN TABLE t1 USING COVERING INDEX i2 (~1000000 rows)

373

2|0|0|SCAN TABLE t2 (~1000000 rows)

374

2|0|0|USE TEMP B-TREE FOR ORDER BY

375

0|0|0|COMPOUND SUBQUERIES 1 AND 2 (EXCEPT)

376

</table></div>

377

378

<h1 id="section_2">2. Sample Code</h1>

379

380

<p>Sometimes, within a large application, it may be inconvenient to modify

381

code to generate EXPLAIN QUERY PLAN commands for the SELECT queries being

382

investigated. From within an interactive debugging session, it may be almost

383

impossible. In these situations, a function similar to the following may

384

be useful. This particular function is passed an SQLite statement handle as an

385

argument and outputs the corresponding EXPLAIN QUERY PLAN report to standard

386

output. Application specific versions may output the report to an application

387

log or similar.

388

389

390

<div class=codeblock style="margin:0 15ex"><table width=100% style="border:1px solid #80a796;padding:0 1ex;background-color:#EEEEEE"><tr><td><pre style="font-size:1.1em">/*

391

** Argument pStmt is a prepared SQL statement. This function compiles

392

** an EXPLAIN QUERY PLAN command to report on the prepared statement,

393

** and prints the report to stdout using printf().

394

395

int printExplainQueryPlan(sqlite3_stmt *pStmt){

396

const char *zSql; /* Input SQL */

397

char *zExplain; /* SQL with EXPLAIN QUERY PLAN prepended */

398

sqlite3_stmt *pExplain; /* Compiled EXPLAIN QUERY PLAN command */

399

int rc; /* Return code from sqlite3_prepare_v2() */

400

401

zSql = sqlite3_sql(pStmt);

402

if( zSql==0 ) return SQLITE_ERROR;

403

404

zExplain = sqlite3_mprintf("EXPLAIN QUERY PLAN %s", zSql);

405

if( zExplain==0 ) return SQLITE_NOMEM;

406

407

rc = sqlite3_prepare_v2(sqlite3_db_handle(pStmt), zExplain, -1, &pExplain, 0);

408

sqlite3_free(zExplain);

409

if( rc!=SQLITE_OK ) return rc;

410

411

while( SQLITE_ROW==sqlite3_step(pExplain) ){

412

int iSelectid = sqlite3_column_int(pExplain, 0);

413

int iOrder = sqlite3_column_int(pExplain, 1);

414

int iFrom = sqlite3_column_int(pExplain, 2);

415

const char *zDetail = (const char *)sqlite3_column_text(pExplain, 3);

416

417

printf("%d %d %d %s\n", iSelectid, iOrder, iFrom, zDetail);

418

}

419

420

return sqlite3_finalize(pExplain);

421

}

422

</table></div>

423